diff --git a/environment.js b/environment.js
index 22ff1cd..833aaf4 100644
--- a/environment.js
+++ b/environment.js
@@ -22,6 +22,10 @@ class Environment {
     return this.map.set(token.lexeme, value)
   }
 
+  setBuiltin(name, func) {
+    this.map.set(name, typeof func === 'function' ? { call: func } : func)
+  }
+
   assign(token, value) {
     if (!this.map.has(token.lexeme)) {
       if (this.enclosing) return this.enclosing.assign(token, value)
diff --git a/index.js b/index.js
index fdbd453..95786a5 100755
--- a/index.js
+++ b/index.js
@@ -44,6 +44,7 @@ const run = (code, environment) => {
       // Error String
       const errorSection = code.substr(e.startCoordinates.index, e.endCoordinates.index)
 
+      // @TODO: Fix this
       // Post Error String
       const backIndex = code.indexOf('\n', e.endCoordinates.index)
       const postErrorStart = backIndex < 0 ? code.length : backIndex
@@ -69,6 +70,7 @@ const runPrompt = () => {
     historySize: +options.history
   })
   const env = new Environment()
+  env.setBuiltin('readFile', (_vars, args) => fs.readFileSync(args[0], 'utf8'))
 
   lineReader.on('line', line => {
     let code = line
@@ -109,6 +111,7 @@ const processOptions = args =>
     .filter(Boolean)
 
 const main = argv => {
+  process.title = 'YALI'
   const args = processOptions(argv.slice(2))
   if (options.debug) console.log(options)
   if (args.length > 1) {
diff --git a/interpreter.js b/interpreter.js
index 347f313..c074486 100644
--- a/interpreter.js
+++ b/interpreter.js
@@ -2,6 +2,7 @@ const { runtimeError } = require('./errors')
 const {
   Binary,
   Unary,
+  Call,
   Literal,
   Logical,
   Var,
@@ -31,6 +32,12 @@ const checkNumber = (token, ...operands) => {
 class Interpreter {
   constructor(environment) {
     this.environment = environment || new Environment()
+    this.environment.setBuiltin('PI', Math.PI)
+    this.environment.setBuiltin('cos', (_vars, args) => Math.cos(args[0]))
+    this.environment.setBuiltin('mod', (_vars, args) => args[0] % args[1])
+    this.environment.setBuiltin('strlen', (_vars, args) => args[0].length)
+    this.environment.setBuiltin('charAt', (_vars, args) => args[0][args[1]])
+    this.environment.setBuiltin('clock', () => new Date().getTime())
   }
 
   interpret(expr) {
@@ -41,6 +48,7 @@ class Interpreter {
     if (expr instanceof Block) return this.visitBlock(expr)
     else if (expr instanceof Assignment) return this.visitAssignment(expr)
     else if (expr instanceof Logical) return this.visitLogical(expr)
+    else if (expr instanceof Call) return this.visitCall(expr)
     else if (expr instanceof While) return this.visitWhile(expr)
     else if (expr instanceof Condition) return this.visitCondition(expr)
     else if (expr instanceof VarStatement) return this.visitVarStatement(expr)
@@ -130,6 +138,18 @@ class Interpreter {
     return value
   }
 
+  visitCall(expr) {
+    const callee = this.evaluate(expr.callee)
+
+    let args = expr.arguments.map(arg => this.evaluate(arg))
+
+    if (!callee.call) {
+      throw runtimeError('Can only call functions and classes', expr.paren)
+    }
+
+    return callee.call(this, args)
+  }
+
   visitUnary(expr) {
     const right = this.evaluate(expr.right)
     switch (expr.operator.type) {
diff --git a/parser.js b/parser.js
index 7b7f32d..5ee1fb2 100644
--- a/parser.js
+++ b/parser.js
@@ -3,6 +3,7 @@ const {
   Binary,
   Unary,
   Var,
+  Call,
   Literal,
   While,
   Grouping,
@@ -75,7 +76,6 @@ class Parser {
     return this.matchBinary('equality', Logical, token.AND)
   }
 
-
   varDeclaration() {
     const name = this.consume(token.IDENTIFIER, 'Expected variable name')
     let initializer = null
@@ -87,8 +87,7 @@ class Parser {
     return new VarStatement(name, initializer)
   }
 
-
-  forStatement () {
+  forStatement() {
     this.consume(token.LEFT_PAREN, 'Expected "(" after "for"')
 
     let init
@@ -204,7 +203,31 @@ class Parser {
       const right = this.unary()
       return new Unary(operator, right)
     }
-    return this.primary()
+    return this.call()
+  }
+
+  call() {
+    let expr = this.primary()
+    while (true) {
+      if (this.match(token.LEFT_PAREN)) {
+        expr = this.finishCall(expr)
+      } else {
+        break
+      }
+    }
+
+    return expr
+  }
+
+  finishCall(callee) {
+    let args = []
+    if (!this.check(token.RIGHT_PAREN)) {
+      do {
+        args.push(this.expression())
+      } while (this.match(token.COMMA))
+    }
+    const paren = this.consume(token.RIGHT_PAREN, 'Unfinished argument list')
+    return new Call(callee, paren, args)
   }
 
   primary() {
diff --git a/types.js b/types.js
index de24d0c..7b7a559 100644
--- a/types.js
+++ b/types.js
@@ -27,7 +27,6 @@ class Var {
   }
 }
 
-// @TODO: Can we express this in a less type-dependant way?
 class Grouping {
   constructor(expression) {
     this.expression = expression
@@ -72,11 +71,31 @@ class While {
   }
 }
 
+class Call {
+  constructor(callee, paren, args) {
+    ;(this.callee = callee), (this.paren = paren)
+    this.arguments = args
+  }
+}
+
+class Callable {
+  constructor(name, func) {
+    this.lexeme = name
+    this.call = func
+  }
+
+  toString() {
+    return '<native fn>'
+  }
+}
+
 module.exports = {
   Var,
   Binary,
   Unary,
   Block,
+  Call,
+  Callable,
   While,
   Literal,
   Logical,