Conjugated linoleic acid (CLA) is the collective name for a group of geometric and positional isomers of octadecadienoic acid (18: 2) with conjugated double bonds. Interest in CLA has increased in the past decade as a result of its potential beneficial health effects on experimental animals. We summarize in this review the occurrence in food, biological effects and typical human consumption of CLA. We also review the efficacy and some problems of CLA supplementation as a weight loss agent, and the findings from animal studies and in vitro studies. Since CLA is biologically produced by the rumen bacteria of ruminant animals, ruminant meat and milk contain relatively higher amounts of CLA. The predominant isomers in those foods are 9-cis, 11-trans CLA. On the other hand, chemically synthesized CLA products which are commercially available in the US generally contain equal amounts of 9-cis, 11-trans and 10-trans, 12-cis CLA. The CLA isomers have been shown in animal models to protect against chemically induced cancer and atherosclerosis, and to reduce body fat. Although CLA appears to reduce fat mass in rodents such as mice and rats, we need more evidence to clarify the fat-reducing effect and safety of CLA in humans.