Although the active component of cannabis Delta(9)-THC was isolated by our group 35 years ago, until recently its mode of action remained obscure. In the last decade it was established that Delta(9)-THC acts through specific receptors - CB1 and CB2 and mimics the physiological activity of endogenous cannabinoids of two types, the best known representatives being arachidonoytethanolamide (anandamide) and 2-arachidonoylglycerol (2-AG). THC is officially used against vomiting caused by cancer chemotherapy and for enhancing appetite, particularly in AIDS patients. Illegally, usually by smoking marijuana, it is used for ameliorating the symptoms of multiple sclerosis, against pain, and in a variety of other diseases. A synthetic cannabinoid, HU-211, is in advanced clinical tests against brain damage caused by closed head injury. It may prove to be valuable against stroke and other neurological diseases.