Squalene is a triterpene that is an intermediate of the cholesterol biosynthesis pathway and it can be obtained from the diet. Olive oil contains 0.2-0.7% squalene. The average intake of squalene is 30 mg/day in the United States, however, when consumption of olive oil is high, the intake of squalene can reach 200-400 mg/day as observed in Mediterranean countries. The decreased risk for various cancers associated with high olive oil consumption may be due to the presence of squalene. Experimental studies have shown that squalene can effectively inhibit chemically-induced colon, lung and skin tumourigenesis in rodents. The protective effect is observed when squalene is given before and/or during carcinogen treatment. The mechanisms involved for the chemopreventive activity of squalene may include inhibition of Ras farnesylation, modulation of carcinogen activation and anti-oxidative activities. However, several factors must be taken into consideration when the evidence for the inhibition of carcinogenesis by squalene is examined, these include the effective dose used and the time of exposure. The information obtained is from animal bioassays and the long-term effects from consuming increased levels of squalene are not known. Although animal studies have enhanced our understanding of the possible action of squalene in decreasing carcinogenesis, one must apply caution in extrapolating the information obtained in animal studies to humans, because of possible species differences. In order to evaluate the overall implications of squalene to human cancer prevention, further studies are needed to fully identify its protective effects, as well as possible detrimental effects.