diff --git a/Developer-Guide.md b/Developer-Guide.md
index bd9d49c..f0c5821 100644
--- a/Developer-Guide.md
+++ b/Developer-Guide.md
@@ -44,6 +44,12 @@ To get JSDoc from GitHub:
 Again, the `jsdoc` executable, located at the root of `jsdoc` repository,
 should be in your `PATH`.
 
+If you are want to modify the WebGL shaders, you'll need
+[glsl-unit](https://code.google.com/p/glsl-unit/).  Get it with git:
+
+    $ git clone https://code.google.com/p/glsl-unit/ build/glsl-unit
+
+
 ### Travis CI
 
 Although not mandatory it is recommended to set up Travis CI for your ol3 fork.