Conjugated linoleic acid (CLA) refers to a class of positional and geometric conjugated isomers of the omega-6 essential fatty acid, linoleic acid (LA). In 1987 anticancerogen properties of CLA were first discovered by Pariza and coworkers coincidentally. Since these early discoveries, an array of purported benefical effects of CLA, such as reduced body fat, enhanced immünity, and anticancer, antidiabetogenic and antiatherogenic properties, on human health have been reported.Since significant amonut of CLA synthesis is not available in humans, the major source of CLA is diet which contains ruminant meats and dairy products.This review addresses the recent studies about the effects of CLA on body composition, immunity, carcinogenesis and diabetes mellitus in both animals and humans and the mechanisms surrounding these effects.