Conjugated linoleic acid (CLA) is a group of positional and geometric isomers of linoleic acid with conjugated double bonds at C9,C11 and C10,C12 positions. Of possible CLA isomers, a naturally occurring CLA isomer is c9,t11-CLA which is produced from linoleic acid by linoleate isomerase from various rumen and lactic bacteria, and mushroom mycelia. Meanwhile, synthetically prepared CLA contained an equal amount of c9,t11-CLA and t10,c12-CLA isomers, and other isomers as minor constituents. CLA was firstly mentioned in 1939 during the elaidinization reaction of linoleic acid. Thereafter, CLA was not an attractant to scientists because it was not scientifically interested any more. However, since the anticarcinogenic action was driven from 7,12-dimethylbenz[a]anthracene (DMBA)-induced mouse skin carcinogenesis in 1987, CLA-related researches were drastically elevated, resulting in approximately 6,100 research papers in literature, so far. CLA exhibited the significant biological activities: anticarcinogenic, antidiabetic, antihypertensive, antiatherosclerotic, body-fat reducing, antioxidative, antiinflammatory, testosterone producing and other activities. Interestingly, two major CLA isomers, c9,t11-CLA and t10,c12-CLA, exhibited different biological activities. Meanwhile, t,t-CLA isomers which is minor constituent of chemically synthesized CLA from linoleic acid exhibited more potent anticarcinogenic activity in carcinogen-induced animal models and cancer cell lines than other CLA isomrs. In the present review, the significant biological activities of CLA were discussed along with historical studies of CLA since 1939.