If you mean the dappling of light on a forest's floor, you can use:

The Cloud Plane Trick.
Create a cloud plane.
Open the MatLab, and make the Diffuse color a dark green. Use channel A for the ambient and transparency. Lowclouds2 works good as a procedural to use in Channel A.
Leave the Mat lab and get a side view of your scene.
Raise/lower the Cloud plane until it is just above the camera's view. You want it low enough to cast shadows without poking through the trees in the middle distance.
Now fiddle with it to your hearts content.