diff --git a/README.md b/README.md
index dd8583a..ac57a2b 100644
--- a/README.md
+++ b/README.md
@@ -50,5 +50,5 @@ To specify a release build in the meson setup step:
 
 ## API
 
-Not yet. `impy.h` is the public interface, but it's subject to massive change.
+See `impy.h`.
 
diff --git a/impy.h b/impy.h
index 1e9fc9d..ce1ffa3 100644
--- a/impy.h
+++ b/impy.h
@@ -15,6 +15,11 @@ extern "C" {
 #include <stdint.h>
 #include <stdbool.h>
 
+
+#define IMPY_VERSION_STRING "0.2"
+// Increment this any time API changes.
+#define IMPY_API_VERSION 2
+
 // The pixelformats we support.
 // X = pad byte
 // A = alpha byte