diff --git a/.gitignore b/.gitignore
index b0d183069..c44c5d5e1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -307,3 +307,6 @@ docs/_site/*
 
 # Ignore Ionide files (https://ionide.io/)
 .ionide
+
+# kdiff/merge files
+*.orig
diff --git a/src/NSubstitute/Callback.cs b/src/NSubstitute/Callback.cs
index 2d7f4dae2..7cf4a1705 100644
--- a/src/NSubstitute/Callback.cs
+++ b/src/NSubstitute/Callback.cs
@@ -18,7 +18,7 @@ public class Callback
         /// </summary>
         /// <param name="doThis"></param>
         /// <returns></returns>
-        public static ConfiguredCallback First(Action<CallInfo> doThis)
+        public static ConfiguredCallback First(Action<ICallInfo> doThis)
         {
             return new ConfiguredCallback().Then(doThis);
         }
@@ -28,7 +28,7 @@ public static ConfiguredCallback First(Action<CallInfo> doThis)
         /// </summary>
         /// <param name="doThis"></param>
         /// <returns></returns>
-        public static Callback Always(Action<CallInfo> doThis)
+        public static Callback Always(Action<ICallInfo> doThis)
         {
             return new ConfiguredCallback().AndAlways(doThis);
         }
@@ -38,7 +38,7 @@ public static Callback Always(Action<CallInfo> doThis)
         /// </summary>
         /// <param name="throwThis"></param>
         /// <returns></returns>
-        public static ConfiguredCallback FirstThrow<TException>(Func<CallInfo, TException> throwThis) where TException : Exception
+        public static ConfiguredCallback FirstThrow<TException>(Func<ICallInfo, TException> throwThis) where TException : Exception
         {
             return new ConfiguredCallback().ThenThrow(throwThis);
         }
@@ -59,7 +59,7 @@ public static ConfiguredCallback FirstThrow<TException>(TException exception) wh
         /// <typeparam name="TException">The type of the exception.</typeparam>
         /// <param name="throwThis">The throw this.</param>
         /// <returns></returns>
-        public static Callback AlwaysThrow<TException>(Func<CallInfo, TException> throwThis) where TException : Exception
+        public static Callback AlwaysThrow<TException>(Func<ICallInfo, TException> throwThis) where TException : Exception
         {
             return new ConfiguredCallback().AndAlways(ToCallback(throwThis));
         }
@@ -75,33 +75,33 @@ public static Callback AlwaysThrow<TException>(TException exception) where TExce
             return AlwaysThrow(_ => exception);
         }
 
-        protected static Action<CallInfo> ToCallback<TException>(Func<CallInfo, TException> throwThis)
+        protected static Action<ICallInfo> ToCallback<TException>(Func<ICallInfo, TException> throwThis)
             where TException : notnull, Exception
         {
             return ci => { if (throwThis != null) throw throwThis(ci); };
         }
 
         internal Callback() { }
-        private readonly ConcurrentQueue<Action<CallInfo>> callbackQueue = new ConcurrentQueue<Action<CallInfo>>();
-        private Action<CallInfo> alwaysDo = x => { };
-        private Action<CallInfo> keepDoing = x => { };
+        private readonly ConcurrentQueue<Action<ICallInfo>> callbackQueue = new ConcurrentQueue<Action<ICallInfo>>();
+        private Action<ICallInfo> alwaysDo = x => { };
+        private Action<ICallInfo> keepDoing = x => { };
 
-        protected void AddCallback(Action<CallInfo> doThis)
+        protected void AddCallback(Action<ICallInfo> doThis)
         {
             callbackQueue.Enqueue(doThis);
         }
 
-        protected void SetAlwaysDo(Action<CallInfo> always)
+        protected void SetAlwaysDo(Action<ICallInfo> always)
         {
             alwaysDo = always ?? (_ => { });
         }
 
-        protected void SetKeepDoing(Action<CallInfo> keep)
+        protected void SetKeepDoing(Action<ICallInfo> keep)
         {
             keepDoing = keep ?? (_ => { });
         }
 
-        public void Call(CallInfo callInfo)
+        public void Call(ICallInfo callInfo)
         {
             try
             {
@@ -113,7 +113,7 @@ public void Call(CallInfo callInfo)
             }
         }
 
-        private void CallFromStack(CallInfo callInfo)
+        private void CallFromStack(ICallInfo callInfo)
         {
             if (callbackQueue.TryDequeue(out var callback))
             {
diff --git a/src/NSubstitute/Callbacks/ConfiguredCallback.cs b/src/NSubstitute/Callbacks/ConfiguredCallback.cs
index 0c32ad881..871b2cf58 100644
--- a/src/NSubstitute/Callbacks/ConfiguredCallback.cs
+++ b/src/NSubstitute/Callbacks/ConfiguredCallback.cs
@@ -13,7 +13,7 @@ internal ConfiguredCallback() { }
         /// <summary>
         /// Perform this action once in chain of called callbacks.
         /// </summary>
-        public ConfiguredCallback Then(Action<CallInfo> doThis)
+        public ConfiguredCallback Then(Action<ICallInfo> doThis)
         {
             AddCallback(doThis);
             return this;
@@ -22,7 +22,7 @@ public ConfiguredCallback Then(Action<CallInfo> doThis)
         /// <summary>
         /// Keep doing this action after the other callbacks have run.
         /// </summary>
-        public EndCallbackChain ThenKeepDoing(Action<CallInfo> doThis)
+        public EndCallbackChain ThenKeepDoing(Action<ICallInfo> doThis)
         {
             SetKeepDoing(doThis);
             return this;
@@ -31,7 +31,7 @@ public EndCallbackChain ThenKeepDoing(Action<CallInfo> doThis)
         /// <summary>
         /// Keep throwing this exception after the other callbacks have run.
         /// </summary>
-        public EndCallbackChain ThenKeepThrowing<TException>(Func<CallInfo, TException> throwThis) where TException : Exception =>
+        public EndCallbackChain ThenKeepThrowing<TException>(Func<ICallInfo, TException> throwThis) where TException : Exception =>
             ThenKeepDoing(ToCallback(throwThis));
 
         /// <summary>
@@ -45,7 +45,7 @@ public EndCallbackChain ThenKeepThrowing<TException>(TException throwThis) where
         /// </summary>
         /// <typeparam name="TException">The type of the exception</typeparam>
         /// <param name="throwThis">Produce the exception to throw for a CallInfo</param>
-        public ConfiguredCallback ThenThrow<TException>(Func<CallInfo, TException> throwThis) where TException : Exception
+        public ConfiguredCallback ThenThrow<TException>(Func<ICallInfo, TException> throwThis) where TException : Exception
         {
             AddCallback(ToCallback(throwThis));
             return this;
@@ -68,7 +68,7 @@ internal EndCallbackChain() { }
         /// Perform the given action for every call.
         /// </summary>
         /// <param name="doThis">The action to perform for every call</param>
-        public Callback AndAlways(Action<CallInfo> doThis)
+        public Callback AndAlways(Action<ICallInfo> doThis)
         {
             SetAlwaysDo(doThis);
             return this;
diff --git a/src/NSubstitute/Core/CallInfo.cs b/src/NSubstitute/Core/CallInfo.cs
index 89aa1b8ea..faf4bc652 100644
--- a/src/NSubstitute/Core/CallInfo.cs
+++ b/src/NSubstitute/Core/CallInfo.cs
@@ -9,72 +9,64 @@
 
 namespace NSubstitute.Core
 {
-    public class CallInfo
+    public class CallInfo : ICallInfo
     {
         private readonly Argument[] _callArguments;
+        private readonly Func<Maybe<object>> _baseResult;
 
-        public CallInfo(Argument[] callArguments)
-        {
+        public CallInfo(Argument[] callArguments, Func<Maybe<object>> baseResult) {
             _callArguments = callArguments;
+            _baseResult = baseResult;
+        }
+
+        protected CallInfo(CallInfo info) : this(info._callArguments, info._baseResult) {
         }
 
         /// <summary>
-        /// Gets the nth argument to this call.
+        /// Call and returns the result from the base implementation of a substitute for a class.
+        /// Will throw an exception if no base implementation exists.
         /// </summary>
-        /// <param name="index">Index of argument</param>
-        /// <returns>The value of the argument at the given index</returns>
-        public object this[int index]
-        {
+        /// <returns>Result from base implementation</returns>
+        /// <exception cref="NoBaseImplementationException">Throws in no base implementation exists</exception>
+        protected object GetBaseResult() {
+            return _baseResult().ValueOr(() => throw new NoBaseImplementationException());
+        }
+
+        /// <inheritdoc/>
+        public object this[int index] {
             get => _callArguments[index].Value;
-            set
-            {
+            set {
                 var argument = _callArguments[index];
                 EnsureArgIsSettable(argument, index, value);
                 argument.Value = value;
             }
         }
 
-        private void EnsureArgIsSettable(Argument argument, int index, object value)
-        {
-            if (!argument.IsByRef)
-            {
+        private void EnsureArgIsSettable(Argument argument, int index, object value) {
+            if (!argument.IsByRef) {
                 throw new ArgumentIsNotOutOrRefException(index, argument.DeclaredType);
             }
 
-            if (value != null && !argument.CanSetValueWithInstanceOf(value.GetType()))
-            {
+            if (value != null && !argument.CanSetValueWithInstanceOf(value.GetType())) {
                 throw new ArgumentSetWithIncompatibleValueException(index, argument.DeclaredType, value.GetType());
             }
         }
 
-        /// <summary>
-        /// Get the arguments passed to this call.
-        /// </summary>
-        /// <returns>Array of all arguments passed to this call</returns>
+        /// <inheritdoc/>
         public object[] Args() => _callArguments.Select(x => x.Value).ToArray();
 
-        /// <summary>
-        /// Gets the types of all the arguments passed to this call.
-        /// </summary>
-        /// <returns>Array of types of all arguments passed to this call</returns>
+        /// <inheritdoc/>
         public Type[] ArgTypes() => _callArguments.Select(x => x.DeclaredType).ToArray();
 
-        /// <summary>
-        /// Gets the argument of type `T` passed to this call. This will throw if there are no arguments
-        /// of this type, or if there is more than one matching argument.
-        /// </summary>
-        /// <typeparam name="T">The type of the argument to retrieve</typeparam>
-        /// <returns>The argument passed to the call, or throws if there is not exactly one argument of this type</returns>
-        public T Arg<T>()
-        {
+        /// <inheritdoc/>
+        public T Arg<T>() {
             T arg;
             if (TryGetArg(x => x.IsDeclaredTypeEqualToOrByRefVersionOf(typeof(T)), out arg)) return arg;
             if (TryGetArg(x => x.IsValueAssignableTo(typeof(T)), out arg)) return arg;
             throw new ArgumentNotFoundException("Can not find an argument of type " + typeof(T).FullName + " to this call.");
         }
 
-        private bool TryGetArg<T>(Func<Argument, bool> condition, [MaybeNullWhen(false)] out T value)
-        {
+        private bool TryGetArg<T>(Func<Argument, bool> condition, [MaybeNullWhen(false)] out T value) {
             value = default;
 
             var matchingArgs = _callArguments.Where(condition);
@@ -85,10 +77,8 @@ private bool TryGetArg<T>(Func<Argument, bool> condition, [MaybeNullWhen(false)]
             return true;
         }
 
-        private void ThrowIfMoreThanOne<T>(IEnumerable<Argument> arguments)
-        {
-            if (arguments.Skip(1).Any())
-            {
+        private void ThrowIfMoreThanOne<T>(IEnumerable<Argument> arguments) {
+            if (arguments.Skip(1).Any()) {
                 throw new AmbiguousArgumentsException(
                     "There is more than one argument of type " + typeof(T).FullName + " to this call.\n" +
                     "The call signature is (" + DisplayTypes(ArgTypes()) + ")\n" +
@@ -97,27 +87,15 @@ private void ThrowIfMoreThanOne<T>(IEnumerable<Argument> arguments)
             }
         }
 
-        /// <summary>
-        /// Gets the argument passed to this call at the specified zero-based position, converted to type `T`.
-        /// This will throw if there are no arguments, if the argument is out of range or if it
-        /// cannot be converted to the specified type.
-        /// </summary>
-        /// <typeparam name="T">The type of the argument to retrieve</typeparam>
-        /// <param name="position">The zero-based position of the argument to retrieve</param>
-        /// <returns>The argument passed to the call, or throws if there is not exactly one argument of this type</returns>
-        public T ArgAt<T>(int position)
-        {
-            if (position >= _callArguments.Length)
-            {
+        /// <inheritdoc/>
+        public T ArgAt<T>(int position) {
+            if (position >= _callArguments.Length) {
                 throw new ArgumentOutOfRangeException(nameof(position), $"There is no argument at position {position}");
             }
 
-            try
-            {
-                return (T) _callArguments[position].Value!;
-            }
-            catch (InvalidCastException)
-            {
+            try {
+                return (T)_callArguments[position].Value!;
+            } catch (InvalidCastException) {
                 throw new InvalidCastException(
                     $"Couldn't convert parameter at position {position} to type {typeof(T).FullName}");
             }
@@ -125,5 +103,8 @@ public T ArgAt<T>(int position)
 
         private static string DisplayTypes(IEnumerable<Type> types) =>
             string.Join(", ", types.Select(x => x.Name).ToArray());
+
+        /// <inheritdoc/>
+        public ICallInfo<T> ForCallReturning<T>() => new CallInfo<T>(this);
     }
 }
diff --git a/src/NSubstitute/Core/CallInfoFactory.cs b/src/NSubstitute/Core/CallInfoFactory.cs
index fa7662d07..a02cc3fa3 100644
--- a/src/NSubstitute/Core/CallInfoFactory.cs
+++ b/src/NSubstitute/Core/CallInfoFactory.cs
@@ -5,7 +5,7 @@ public class CallInfoFactory : ICallInfoFactory
         public CallInfo Create(ICall call)
         {
             var arguments = GetArgumentsFromCall(call);
-            return new CallInfo(arguments);
+            return new CallInfo(arguments, () => call.TryCallBase());
         }
 
         private static Argument[] GetArgumentsFromCall(ICall call)
diff --git a/src/NSubstitute/Core/CallInfoWithReturns.cs b/src/NSubstitute/Core/CallInfoWithReturns.cs
new file mode 100644
index 000000000..5eb12aefd
--- /dev/null
+++ b/src/NSubstitute/Core/CallInfoWithReturns.cs
@@ -0,0 +1,16 @@
+namespace NSubstitute.Core
+{
+    /// <summary>
+    /// Information for a call that returns a value of type <c>T</c>.
+    /// </summary>
+    /// <typeparam name="T"></typeparam>
+    public class CallInfo<T> : CallInfo, ICallInfo<T>
+    {
+        internal CallInfo(CallInfo info) : base(info) {
+        }
+
+        public T BaseResult() {
+            return (T)GetBaseResult();
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/NSubstitute/Core/IReturn.cs b/src/NSubstitute/Core/IReturn.cs
index 0901f2820..817e25008 100644
--- a/src/NSubstitute/Core/IReturn.cs
+++ b/src/NSubstitute/Core/IReturn.cs
@@ -9,13 +9,13 @@ namespace NSubstitute.Core
 {
     public interface IReturn
     {
-        object? ReturnFor(CallInfo info);
+        object? ReturnFor(ICallInfo info);
         Type? TypeOrNull();
         bool CanBeAssignedTo(Type t);
     }
 
     /// <summary>
-    /// Performance optimization. Allows to not construct <see cref="CallInfo"/> if configured result doesn't depend on it.
+    /// Performance optimization. Allows to not construct <see cref="ICallInfo"/> if configured result doesn't depend on it.
     /// </summary>
     internal interface ICallIndependentReturn
     {
@@ -32,28 +32,28 @@ public ReturnValue(object? value)
         }
 
         public object? GetReturnValue() => _value;
-        public object? ReturnFor(CallInfo info) => GetReturnValue();
+        public object? ReturnFor(ICallInfo info) => GetReturnValue();
         public Type? TypeOrNull() => _value?.GetType();
         public bool CanBeAssignedTo(Type t) => _value.IsCompatibleWith(t);
     }
 
     public class ReturnValueFromFunc<T> : IReturn
     {
-        private readonly Func<CallInfo, T?> _funcToReturnValue;
+        private readonly Func<ICallInfo<T>, T?> _funcToReturnValue;
 
-        public ReturnValueFromFunc(Func<CallInfo, T?>? funcToReturnValue)
+        public ReturnValueFromFunc(Func<ICallInfo<T>, T?>? funcToReturnValue)
         {
             _funcToReturnValue = funcToReturnValue ?? ReturnNull();
         }
 
-        public object? ReturnFor(CallInfo info) => _funcToReturnValue(info);
-        public Type TypeOrNull() => typeof (T);
-        public bool CanBeAssignedTo(Type t) => typeof (T).IsAssignableFrom(t);
+        public object? ReturnFor(ICallInfo info) => _funcToReturnValue(info.ForCallReturning<T>());
+        public Type TypeOrNull() => typeof(T);
+        public bool CanBeAssignedTo(Type t) => typeof(T).IsAssignableFrom(t);
 
-        private static Func<CallInfo, T?> ReturnNull()
+        private static Func<ICallInfo, T?> ReturnNull()
         {
             if (typeof(T).GetTypeInfo().IsValueType) throw new CannotReturnNullForValueType(typeof(T));
-            return x => default(T);
+            return x => default;
         }
     }
 
@@ -69,28 +69,31 @@ public ReturnMultipleValues(T?[] values)
         }
 
         public object? GetReturnValue() => GetNext();
-        public object? ReturnFor(CallInfo info) => GetReturnValue();
-        public Type TypeOrNull() => typeof (T);
-        public bool CanBeAssignedTo(Type t) => typeof (T).IsAssignableFrom(t);
+        public object? ReturnFor(ICallInfo info) => GetReturnValue();
+        public Type TypeOrNull() => typeof(T);
+        public bool CanBeAssignedTo(Type t) => typeof(T).IsAssignableFrom(t);
 
         private T? GetNext() => _valuesToReturn.TryDequeue(out var nextResult) ? nextResult : _lastValue;
     }
 
     public class ReturnMultipleFuncsValues<T> : IReturn
     {
-        private readonly ConcurrentQueue<Func<CallInfo, T?>> _funcsToReturn;
-        private readonly Func<CallInfo, T?> _lastFunc;
+        private readonly ConcurrentQueue<Func<ICallInfo<T>, T?>> _funcsToReturn;
+        private readonly Func<ICallInfo<T>, T?> _lastFunc;
 
-        public ReturnMultipleFuncsValues(Func<CallInfo, T?>[] funcs)
+        public ReturnMultipleFuncsValues(Func<ICallInfo<T>, T?>[] funcs)
         {
-            _funcsToReturn = new ConcurrentQueue<Func<CallInfo, T?>>(funcs);
+            _funcsToReturn = new ConcurrentQueue<Func<ICallInfo<T>, T?>>(funcs);
             _lastFunc = funcs.Last();
         }
 
-        public object? ReturnFor(CallInfo info) => GetNext(info);
-        public Type TypeOrNull() => typeof (T);
-        public bool CanBeAssignedTo(Type t) => typeof (T).IsAssignableFrom(t);
+        public object? ReturnFor(ICallInfo info) => GetNext(info);
+        public Type TypeOrNull() => typeof(T);
+        public bool CanBeAssignedTo(Type t) => typeof(T).IsAssignableFrom(t);
 
-        private T? GetNext(CallInfo info) => _funcsToReturn.TryDequeue(out var nextFunc) ? nextFunc(info) : _lastFunc(info);
+        private T? GetNext(ICallInfo info) =>
+            _funcsToReturn.TryDequeue(out var nextFunc)
+            ? nextFunc(info.ForCallReturning<T>())
+            : _lastFunc(info.ForCallReturning<T>());
     }
 }
\ No newline at end of file
diff --git a/src/NSubstitute/Core/WhenCalled.cs b/src/NSubstitute/Core/WhenCalled.cs
index fbc80c24d..daf9e4104 100644
--- a/src/NSubstitute/Core/WhenCalled.cs
+++ b/src/NSubstitute/Core/WhenCalled.cs
@@ -29,7 +29,7 @@ public WhenCalled(ISubstitutionContext context, T substitute, Action<T> call, Ma
         /// Perform this action when called.
         /// </summary>
         /// <param name="callbackWithArguments"></param>
-        public void Do(Action<CallInfo> callbackWithArguments)
+        public void Do(Action<ICallInfo> callbackWithArguments)
         {
             _threadContext.SetNextRoute(_callRouter, x => _routeFactory.DoWhenCalled(x, callbackWithArguments, _matchArgs));
             _call(_substitute);
@@ -82,7 +82,7 @@ public void Throw(Exception exception) =>
         /// <summary>
         /// Throw an exception generated by the specified function when called.
         /// </summary>
-        public void Throw(Func<CallInfo, Exception> createException) =>
+        public void Throw(Func<ICallInfo, Exception> createException) =>
             Do(ci => throw createException(ci));
     }
 }
\ No newline at end of file
diff --git a/src/NSubstitute/Exceptions/NoBaseImplementationException.cs b/src/NSubstitute/Exceptions/NoBaseImplementationException.cs
new file mode 100644
index 000000000..3ab623ff1
--- /dev/null
+++ b/src/NSubstitute/Exceptions/NoBaseImplementationException.cs
@@ -0,0 +1,11 @@
+namespace NSubstitute.Exceptions
+{
+    public class NoBaseImplementationException : SubstituteException
+    {
+        private const string Explanation =
+            "Cannot call the base method as the base method implementation is missing. " +
+            "You can call base method only if you create a class substitute and the method is not abstract.";
+
+        public NoBaseImplementationException() : base(Explanation) { }
+    }
+}
diff --git a/src/NSubstitute/Extensions/ExceptionExtensions.cs b/src/NSubstitute/Extensions/ExceptionExtensions.cs
index 60f3b5339..15004dc88 100644
--- a/src/NSubstitute/Extensions/ExceptionExtensions.cs
+++ b/src/NSubstitute/Extensions/ExceptionExtensions.cs
@@ -35,7 +35,7 @@ public static ConfiguredCall Throws<TException>(this object value)
         /// <param name="value"></param>
         /// <param name="createException">Func creating exception object</param>
         /// <returns></returns>
-        public static ConfiguredCall Throws(this object value, Func<CallInfo, Exception> createException) =>
+        public static ConfiguredCall Throws(this object value, Func<ICallInfo, Exception> createException) =>
             value.Returns(ci => throw createException(ci));
 
         /// <summary>
@@ -65,7 +65,7 @@ public static ConfiguredCall ThrowsForAnyArgs<TException>(this object value)
         /// <param name="value"></param>
         /// <param name="createException">Func creating exception object</param>
         /// <returns></returns>
-        public static ConfiguredCall ThrowsForAnyArgs(this object value, Func<CallInfo, Exception> createException) =>
+        public static ConfiguredCall ThrowsForAnyArgs(this object value, Func<ICallInfo, Exception> createException) =>
             value.ReturnsForAnyArgs(ci => throw createException(ci));
     }
 }
diff --git a/src/NSubstitute/Extensions/ReturnsForAllExtensions.cs b/src/NSubstitute/Extensions/ReturnsForAllExtensions.cs
index 302033d4c..81df863c6 100644
--- a/src/NSubstitute/Extensions/ReturnsForAllExtensions.cs
+++ b/src/NSubstitute/Extensions/ReturnsForAllExtensions.cs
@@ -31,7 +31,7 @@ public static void ReturnsForAll<T>(this object substitute, T returnThis)
         /// <param name="substitute"></param>
         /// <param name="returnThis"></param>
         /// <returns></returns>
-        public static void ReturnsForAll<T>(this object substitute, Func<CallInfo, T> returnThis)
+        public static void ReturnsForAll<T>(this object substitute, Func<ICallInfo, T> returnThis)
         {
             if (substitute == null) throw new NullSubstituteReferenceException();
 
diff --git a/src/NSubstitute/ICallInfo.cs b/src/NSubstitute/ICallInfo.cs
new file mode 100644
index 000000000..2a72b572f
--- /dev/null
+++ b/src/NSubstitute/ICallInfo.cs
@@ -0,0 +1,68 @@
+using System;
+
+namespace NSubstitute
+{
+    /// <summary>
+    /// Access information on arguments for call.
+    /// </summary>
+    public interface ICallInfo
+    {
+        /// <summary>
+        /// Gets the nth argument to this call.
+        /// </summary>
+        /// <param name="index">Index of argument</param>
+        /// <returns>The value of the argument at the given index</returns>
+        object this[int index] { get; set; }
+
+        /// <summary>
+        /// Gets the argument of type `T` passed to this call. This will throw if there are no arguments
+        /// of this type, or if there is more than one matching argument.
+        /// </summary>
+        /// <typeparam name="T">The type of the argument to retrieve</typeparam>
+        /// <returns>The argument passed to the call, or throws if there is not exactly one argument of this type</returns>
+        T Arg<T>();
+
+        /// <summary>
+        /// Gets the argument passed to this call at the specified zero-based position, converted to type `T`.
+        /// This will throw if there are no arguments, if the argument is out of range or if it
+        /// cannot be converted to the specified type.
+        /// </summary>
+        /// <typeparam name="T">The type of the argument to retrieve</typeparam>
+        /// <param name="position">The zero-based position of the argument to retrieve</param>
+        /// <returns>The argument passed to the call, or throws if there is not exactly one argument of this type</returns>
+        T ArgAt<T>(int position);
+
+        /// <summary>
+        /// Get the arguments passed to this call.
+        /// </summary>
+        /// <returns>Array of all arguments passed to this call</returns>
+        object[] Args();
+
+        /// <summary>
+        /// Gets the types of all the arguments passed to this call.
+        /// </summary>
+        /// <returns>Array of types of all arguments passed to this call</returns>
+        Type[] ArgTypes();
+
+        /// <summary>
+        /// If we are sure this call returns a value of type <typeparamref name="T"/>, return an
+        /// <see cref="ICallInfo{T}"/> that allows us to access the <see cref="ICallInfo{T}.BaseResult"/>.
+        ///
+        /// This will not be checked by the compiler, so if this method is misused the resulting <see cref="ICallInfo{T}"/>
+        /// may throw <see cref="InvalidCastException"/>.
+        /// </summary>
+        /// <typeparam name="T"></typeparam>
+        /// <returns></returns>
+        ICallInfo<T> ForCallReturning<T>();
+    }
+
+    public interface ICallInfo<T> : ICallInfo
+    {
+        /// <summary>
+        /// Calls the base implementation and attempts to cast the result to <typeparamref name="T"/>.
+        /// </summary>
+        /// <returns>Result from base (non-substituted) implementation of call</returns>
+        /// <exception cref="InvalidCastException" />
+        T BaseResult();
+    }
+}
\ No newline at end of file
diff --git a/src/NSubstitute/Routing/Handlers/ReturnFromAndConfigureDynamicCall.cs b/src/NSubstitute/Routing/Handlers/ReturnFromAndConfigureDynamicCall.cs
index e4fadd2c4..70a720dad 100644
--- a/src/NSubstitute/Routing/Handlers/ReturnFromAndConfigureDynamicCall.cs
+++ b/src/NSubstitute/Routing/Handlers/ReturnFromAndConfigureDynamicCall.cs
@@ -55,7 +55,7 @@ public ConfiguredCall Returns<T>(T? returnThis, params T?[] returnThese)
                 return default(T).Returns(returnThis, returnThese);
             }
 
-            public ConfiguredCall Returns<T>(Func<CallInfo, T?> returnThis, params Func<CallInfo, T?>[] returnThese)
+            public ConfiguredCall Returns<T>(Func<ICallInfo, T?> returnThis, params Func<ICallInfo, T?>[] returnThese)
             {
                 return default(T).Returns(returnThis, returnThese);
             }
@@ -65,7 +65,7 @@ public ConfiguredCall ReturnsForAnyArgs<T>(T? returnThis, params T?[] returnThes
                 return default(T).ReturnsForAnyArgs(returnThis, returnThese);
             }
 
-            public ConfiguredCall ReturnsForAnyArgs<T>(Func<CallInfo, T?> returnThis, params Func<CallInfo, T?>[] returnThese)
+            public ConfiguredCall ReturnsForAnyArgs<T>(Func<ICallInfo, T?> returnThis, params Func<ICallInfo, T?>[] returnThese)
             {
                 return default(T).ReturnsForAnyArgs(returnThis, returnThese);
             }
diff --git a/src/NSubstitute/SubstituteExtensions.Returns.Task.cs b/src/NSubstitute/SubstituteExtensions.Returns.Task.cs
index 57388dfeb..6d8ba1cda 100644
--- a/src/NSubstitute/SubstituteExtensions.Returns.Task.cs
+++ b/src/NSubstitute/SubstituteExtensions.Returns.Task.cs
@@ -33,14 +33,14 @@ public static ConfiguredCall Returns<T>(this Task<T> value, T returnThis, params
         /// <param name="value"></param>
         /// <param name="returnThis">Function to calculate the return value</param>
         /// <param name="returnThese">Optionally use these functions next</param>
-        public static ConfiguredCall Returns<T>(this Task<T> value, Func<CallInfo, T> returnThis, params Func<CallInfo, T>[] returnThese)
+        public static ConfiguredCall Returns<T>(this Task<T> value, Func<ICallInfo, T> returnThis, params Func<ICallInfo, T>[] returnThese)
         {
             ReThrowOnNSubstituteFault(value);
 
             var wrappedFunc = WrapFuncInTask(returnThis);
             var wrappedReturnThese = returnThese.Length > 0 ? returnThese.Select(WrapFuncInTask).ToArray() : null;
 
-            return ConfigureReturn(MatchArgs.AsSpecifiedInCall, wrappedFunc, wrappedReturnThese);
+            return ConfigureFuncReturn(MatchArgs.AsSpecifiedInCall, wrappedFunc, wrappedReturnThese);
         }
 
         /// <summary>
@@ -65,14 +65,14 @@ public static ConfiguredCall ReturnsForAnyArgs<T>(this Task<T> value, T returnTh
         /// <param name="value"></param>
         /// <param name="returnThis">Function to calculate the return value</param>
         /// <param name="returnThese">Optionally use these functions next</param>
-        public static ConfiguredCall ReturnsForAnyArgs<T>(this Task<T> value, Func<CallInfo, T> returnThis, params Func<CallInfo, T>[] returnThese)
+        public static ConfiguredCall ReturnsForAnyArgs<T>(this Task<T> value, Func<ICallInfo, T> returnThis, params Func<ICallInfo, T>[] returnThese)
         {
             ReThrowOnNSubstituteFault(value);
 
             var wrappedFunc = WrapFuncInTask(returnThis);
             var wrappedReturnThese = returnThese.Length > 0 ? returnThese.Select(WrapFuncInTask).ToArray() : null;
 
-            return ConfigureReturn(MatchArgs.Any, wrappedFunc, wrappedReturnThese);
+            return ConfigureFuncReturn(MatchArgs.Any, wrappedFunc, wrappedReturnThese);
         }
 
 #nullable restore
@@ -86,7 +86,7 @@ private static void ReThrowOnNSubstituteFault<T>(Task<T?> task)
 
         private static Task<T?> CompletedTask<T>(T? result) => Task.FromResult(result);
 
-        private static Func<CallInfo, Task<T?>> WrapFuncInTask<T>(Func<CallInfo, T> returnThis) =>
+        private static Func<ICallInfo, Task<T?>> WrapFuncInTask<T>(Func<ICallInfo, T> returnThis) =>
             x => CompletedTask(returnThis(x));
     }
 }
\ No newline at end of file
diff --git a/src/NSubstitute/SubstituteExtensions.Returns.ValueTask.cs b/src/NSubstitute/SubstituteExtensions.Returns.ValueTask.cs
index d899f0f7d..91483835e 100644
--- a/src/NSubstitute/SubstituteExtensions.Returns.ValueTask.cs
+++ b/src/NSubstitute/SubstituteExtensions.Returns.ValueTask.cs
@@ -33,14 +33,14 @@ public static ConfiguredCall Returns<T>(this ValueTask<T> value, T returnThis, p
         /// <param name="value"></param>
         /// <param name="returnThis">Function to calculate the return value</param>
         /// <param name="returnThese">Optionally use these functions next</param>
-        public static ConfiguredCall Returns<T>(this ValueTask<T> value, Func<CallInfo, T> returnThis, params Func<CallInfo, T>[] returnThese)
+        public static ConfiguredCall Returns<T>(this ValueTask<T> value, Func<ICallInfo, T> returnThis, params Func<ICallInfo, T>[] returnThese)
         {
             ReThrowOnNSubstituteFault(value);
 
             var wrappedFunc = WrapFuncInValueTask(returnThis);
             var wrappedReturnThese = returnThese.Length > 0 ? returnThese.Select(WrapFuncInValueTask).ToArray() : null;
 
-            return ConfigureReturn(MatchArgs.AsSpecifiedInCall, wrappedFunc, wrappedReturnThese);
+            return ConfigureFuncReturn(MatchArgs.AsSpecifiedInCall, wrappedFunc, wrappedReturnThese);
         }
 
         /// <summary>
@@ -65,14 +65,14 @@ public static ConfiguredCall ReturnsForAnyArgs<T>(this ValueTask<T> value, T ret
         /// <param name="value"></param>
         /// <param name="returnThis">Function to calculate the return value</param>
         /// <param name="returnThese">Optionally use these functions next</param>
-        public static ConfiguredCall ReturnsForAnyArgs<T>(this ValueTask<T> value, Func<CallInfo, T> returnThis, params Func<CallInfo, T>[] returnThese)
+        public static ConfiguredCall ReturnsForAnyArgs<T>(this ValueTask<T> value, Func<ICallInfo, T> returnThis, params Func<ICallInfo, T>[] returnThese)
         {
             ReThrowOnNSubstituteFault(value);
 
             var wrappedFunc = WrapFuncInValueTask(returnThis);
             var wrappedReturnThese = returnThese.Length > 0 ? returnThese.Select(WrapFuncInValueTask).ToArray() : null;
 
-            return ConfigureReturn(MatchArgs.Any, wrappedFunc, wrappedReturnThese);
+            return ConfigureFuncReturn(MatchArgs.Any, wrappedFunc, wrappedReturnThese);
         }
 
 #nullable restore
@@ -86,7 +86,7 @@ private static void ReThrowOnNSubstituteFault<T>(ValueTask<T?> task)
 
         private static ValueTask<T?> CompletedValueTask<T>(T? result) => new(result);
 
-        private static Func<CallInfo, ValueTask<T?>> WrapFuncInValueTask<T>(Func<CallInfo, T> returnThis) =>
+        private static Func<ICallInfo, ValueTask<T?>> WrapFuncInValueTask<T>(Func<ICallInfo, T> returnThis) =>
             x => CompletedValueTask(returnThis(x));
     }
 }
\ No newline at end of file
diff --git a/src/NSubstitute/SubstituteExtensions.Returns.cs b/src/NSubstitute/SubstituteExtensions.Returns.cs
index 0e959ebd4..13bcbd57f 100644
--- a/src/NSubstitute/SubstituteExtensions.Returns.cs
+++ b/src/NSubstitute/SubstituteExtensions.Returns.cs
@@ -24,8 +24,8 @@ public static ConfiguredCall Returns<T>(this T value, T returnThis, params T[] r
         /// <param name="value"></param>
         /// <param name="returnThis">Function to calculate the return value</param>
         /// <param name="returnThese">Optionally use these functions next</param>
-        public static ConfiguredCall Returns<T>(this T value, Func<CallInfo, T> returnThis, params Func<CallInfo, T>[] returnThese) =>
-            ConfigureReturn(MatchArgs.AsSpecifiedInCall, returnThis, returnThese);
+        public static ConfiguredCall Returns<T>(this T value, Func<ICallInfo<T>, T> returnThis, params Func<ICallInfo<T>, T>[] returnThese) =>
+            ConfigureFuncReturn(MatchArgs.AsSpecifiedInCall, returnThis, returnThese);
 
         /// <summary>
         /// Set a return value for this call made with any arguments.
@@ -43,8 +43,8 @@ public static ConfiguredCall ReturnsForAnyArgs<T>(this T value, T returnThis, pa
         /// <param name="returnThis">Function to calculate the return value</param>
         /// <param name="returnThese">Optionally use these functions next</param>
         /// <returns></returns>
-        public static ConfiguredCall ReturnsForAnyArgs<T>(this T value, Func<CallInfo, T> returnThis, params Func<CallInfo, T>[] returnThese) =>
-            ConfigureReturn(MatchArgs.Any, returnThis, returnThese);
+        public static ConfiguredCall ReturnsForAnyArgs<T>(this T value, Func<ICallInfo<T>, T> returnThis, params Func<ICallInfo<T>, T>[] returnThese) =>
+            ConfigureFuncReturn(MatchArgs.Any, returnThis, returnThese);
 
 #nullable restore
         private static ConfiguredCall ConfigureReturn<T>(MatchArgs matchArgs, T? returnThis, T?[]? returnThese)
@@ -64,7 +64,7 @@ private static ConfiguredCall ConfigureReturn<T>(MatchArgs matchArgs, T? returnT
                 .LastCallShouldReturn(returnValue, matchArgs);
         }
 
-        private static ConfiguredCall ConfigureReturn<T>(MatchArgs matchArgs, Func<CallInfo, T?> returnThis, Func<CallInfo, T?>[]? returnThese)
+        private static ConfiguredCall ConfigureFuncReturn<T>(MatchArgs matchArgs, Func<ICallInfo<T>, T?> returnThis, Func<ICallInfo<T>, T?>[]? returnThese)
         {
             IReturn returnValue;
             if (returnThese == null || returnThese.Length == 0)
diff --git a/src/NSubstitute/SubstituteExtensions.When.Task.cs b/src/NSubstitute/SubstituteExtensions.When.Task.cs
index 79a20a2fa..37231c93d 100644
--- a/src/NSubstitute/SubstituteExtensions.When.Task.cs
+++ b/src/NSubstitute/SubstituteExtensions.When.Task.cs
@@ -11,7 +11,7 @@ public static partial class SubstituteExtensions
     {
         /// <summary>
         /// Perform an action when this member is called.
-        /// Must be followed by <see cref="WhenCalled{TSubstitute}.Do(Action{CallInfo})"/> to provide the callback.
+        /// Must be followed by <see cref="WhenCalled{TSubstitute}.Do(Action{ICallInfo})"/> to provide the callback.
         /// </summary>
         public static WhenCalled<T> When<T>(this T substitute, Func<T, Task> substituteCall) where T : class
         {
@@ -20,7 +20,7 @@ public static WhenCalled<T> When<T>(this T substitute, Func<T, Task> substituteC
 
         /// <summary>
         /// Perform an action when this member is called with any arguments.
-        /// Must be followed by <see cref="WhenCalled{TSubstitute}.Do(Action{CallInfo})"/> to provide the callback.
+        /// Must be followed by <see cref="WhenCalled{TSubstitute}.Do(Action{ICallInfo})"/> to provide the callback.
         /// </summary>
         public static WhenCalled<T> WhenForAnyArgs<T>(this T substitute, Func<T, Task> substituteCall) where T : class
         {
diff --git a/src/NSubstitute/SubstituteExtensions.When.ValueTask.cs b/src/NSubstitute/SubstituteExtensions.When.ValueTask.cs
index 7759804da..f0d1ece1c 100644
--- a/src/NSubstitute/SubstituteExtensions.When.ValueTask.cs
+++ b/src/NSubstitute/SubstituteExtensions.When.ValueTask.cs
@@ -11,7 +11,7 @@ public static partial class SubstituteExtensions
     {
         /// <summary>
         /// Perform an action when this member is called.
-        /// Must be followed by <see cref="WhenCalled{TSubstitute}.Do(Action{CallInfo})"/> to provide the callback.
+        /// Must be followed by <see cref="WhenCalled{TSubstitute}.Do(Action{ICallInfo})"/> to provide the callback.
         /// </summary>
         public static WhenCalled<TSubstitute> When<TSubstitute, TResult>(this TSubstitute substitute,
             Func<TSubstitute, ValueTask<TResult>> substituteCall) where TSubstitute : class
@@ -21,7 +21,7 @@ public static WhenCalled<TSubstitute> When<TSubstitute, TResult>(this TSubstitut
 
         /// <summary>
         /// Perform an action when this member is called with any arguments.
-        /// Must be followed by <see cref="WhenCalled{TSubstitute}.Do(Action{CallInfo})"/> to provide the callback.
+        /// Must be followed by <see cref="WhenCalled{TSubstitute}.Do(Action{ICallInfo})"/> to provide the callback.
         /// </summary>
         public static WhenCalled<TSubstitute> WhenForAnyArgs<TSubstitute, TResult>(this TSubstitute substitute,
             Func<TSubstitute, ValueTask<TResult>> substituteCall) where TSubstitute : class
diff --git a/src/NSubstitute/SubstituteExtensions.When.cs b/src/NSubstitute/SubstituteExtensions.When.cs
index 055e2aa69..0da4b33c6 100644
--- a/src/NSubstitute/SubstituteExtensions.When.cs
+++ b/src/NSubstitute/SubstituteExtensions.When.cs
@@ -11,7 +11,7 @@ public static partial class SubstituteExtensions
     {
         /// <summary>
         /// Perform an action when this member is called.
-        /// Must be followed by <see cref="WhenCalled{T}.Do(Action{CallInfo})"/> to provide the callback.
+        /// Must be followed by <see cref="WhenCalled{T}.Do(Action{ICallInfo})"/> to provide the callback.
         /// </summary>
         public static WhenCalled<T> When<T>(this T substitute, Action<T> substituteCall) where T : class
         {
@@ -20,7 +20,7 @@ public static WhenCalled<T> When<T>(this T substitute, Action<T> substituteCall)
 
         /// <summary>
         /// Perform an action when this member is called with any arguments.
-        /// Must be followed by <see cref="WhenCalled{T}.Do(Action{CallInfo})"/> to provide the callback.
+        /// Must be followed by <see cref="WhenCalled{T}.Do(Action{ICallInfo})"/> to provide the callback.
         /// </summary>
         public static WhenCalled<T> WhenForAnyArgs<T>(this T substitute, Action<T> substituteCall) where T : class
         {
diff --git a/tests/NSubstitute.Acceptance.Specs/CallbackCalling.cs b/tests/NSubstitute.Acceptance.Specs/CallbackCalling.cs
index 95beadbee..e24dd47bb 100644
--- a/tests/NSubstitute.Acceptance.Specs/CallbackCalling.cs
+++ b/tests/NSubstitute.Acceptance.Specs/CallbackCalling.cs
@@ -100,7 +100,7 @@ public void Throw_exception_when_Throw_with_specific_exception()
         [Test]
         public void Throw_exception_when_Throw_with_exception_generator()
         {
-            Func<CallInfo, Exception> createException = ci => new ArgumentException("Argument: " + ci.Args()[0]);
+            Func<ICallInfo, Exception> createException = ci => new ArgumentException("Argument: " + ci.Args()[0]);
             int called = 0;
             _something.When(x => x.Echo(Arg.Any<int>())).Do(Callback.Always(x => called++));
             _something.When(x => x.Echo(Arg.Any<int>())).Do(Callback.AlwaysThrow(createException));
diff --git a/tests/NSubstitute.Acceptance.Specs/ReturnFromBase.cs b/tests/NSubstitute.Acceptance.Specs/ReturnFromBase.cs
new file mode 100644
index 000000000..b17d73bac
--- /dev/null
+++ b/tests/NSubstitute.Acceptance.Specs/ReturnFromBase.cs
@@ -0,0 +1,52 @@
+using System;
+using NSubstitute.Exceptions;
+using NUnit.Framework;
+
+namespace NSubstitute.Acceptance.Specs
+{
+    public class ReturnFromBase
+    {
+        public class Sample
+        {
+            public virtual string RepeatButLouder(string s) => s + "!";
+            public virtual void VoidMethod() { }
+        }
+
+        public abstract class SampleWithAbstractMethod
+        {
+            public abstract string NoBaseImplementation();
+        }
+
+        public interface ISample
+        {
+            string InterfaceMethod();
+        }
+
+        [Test]
+        public void UseBaseInReturn() {
+            var sub = Substitute.For<Sample>();
+            sub.RepeatButLouder(Arg.Any<string>()).Returns(x => x.BaseResult() + "?");
+
+            Assert.AreEqual("Hi!?", sub.RepeatButLouder("Hi"));
+        }
+
+        [Test]
+        public void CallWithNoBaseImplementation() {
+            var sub = Substitute.For<SampleWithAbstractMethod>();
+            sub.NoBaseImplementation().Returns(x => x.BaseResult());
+
+            Assert.Throws<NoBaseImplementationException>(() =>
+                sub.NoBaseImplementation()
+            );
+        }
+
+        [Test]
+        public void CallBaseForInterface() {
+            var sub = Substitute.For<ISample>();
+            sub.InterfaceMethod().Returns(x => x.BaseResult());
+            Assert.Throws<NoBaseImplementationException>(() =>
+                sub.InterfaceMethod()
+            );
+        }
+    }
+}
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/WhenCalledDo.cs b/tests/NSubstitute.Acceptance.Specs/WhenCalledDo.cs
index 6778947c1..7029fb3d7 100644
--- a/tests/NSubstitute.Acceptance.Specs/WhenCalledDo.cs
+++ b/tests/NSubstitute.Acceptance.Specs/WhenCalledDo.cs
@@ -99,7 +99,7 @@ public void Throw_exception_when_Throw_with_specific_exception()
         [Test]
         public void Throw_exception_when_Throw_with_exception_generator()
         {
-            Func<CallInfo, Exception> createException = ci => new ArgumentException("Argument: " + ci.Args()[0]);
+            Func<ICallInfo, Exception> createException = ci => new ArgumentException("Argument: " + ci.Args()[0]);
             int called = 0;
             _something.When(x => x.Echo(Arg.Any<int>())).Do(x => called++);
             _something.When(x => x.Echo(Arg.Any<int>())).Throw(createException);