Why do pets eat grass?

I get this question a lot and many clients believe that their pet is ill or nutritionally deficient when they eat grass.  I did some research on this topic and found an article by Benjamin L. Hart, DVM, PhD, DACVB who was a distinguished professor at UC Davis at the time of the article I found. He cited several studies that showed that wolves and cougars also eat grass routinely.  He followed up with some surveys of the patient population there and found that a high percentage of normal pets eat grass with no vomiting or illness associated with it.  He feels that the tendency to consume plant material is a trait inherited from wild ancestors and is not abnormal.  However, he does caution (as do I) that if your pet is vomiting +/- plant eating, he or she should be examined by a veterinarian.  Also, use caution with pets eating chemically treated lawns.