There are multiple lines of evidence that a variety of natural fatty acids are effective in health promotion. Among these fatty acids, conjugated linoleic acid (CLA)--a collective term referring to a mixture of positional and geometric isomers of linoleic acid (LA, cis-9, cis-12-octadecadienoic acid)--is currently under intensive investigation due to its health-promotion potential. The antitumor activity of CLA is of special interest, since it shows inhibitory effects against multistage carcinogenesis at relatively low dietary levels. Many studies using in vivo and in vitro models have shown that CLA suppresses the development of multistage carcinogenesis at different sites. The research to date on CLA has provided a vast amount of information about the mechanism on how CLA functions in the prevention of cancer. This article discusses characteristics of CLA in the prevention of cancer in both in vivo and in vitro studies and the possible underlying chemoprevention mechanisms.