The Gluten-Free, Casein-Free Diet is the basic "starter diet" of the autism world.

The theory behind it is that children with autism tend to have weaker immune systems and damage to the lining of the gut. This "leaky gut" allows undigested particles of gluten and casein to pass through the gut wall and into the bloodstream where it reaches the brain and has an opiate-like effect.

More details can be found HERE.

THIS CHART is helpful when implementing a GFCF diet. The third column indicates whether the food is GFCF.