I think the link Nancy provided,
The Dog Food Project - Myths about Dog Nutrition, is excellent. What she says about grains makes a lot of sense: basically, that dogs can benefit from grains, but they have to be ground, cooked, etc so the dog can digest them. If you compare a cat, a dog, a human, and a cow, you can see from the teeth and the digestive system a progression that is just for meat (lots of sharp teeth for ripping and shredding) to one designed just for plants (broad teeth, three stomachs).