n-3 Polyunsaturated fatty acids are promising molecules in cancer prevention and the potentiation of cancer treatment. Recent studies have highlighted the importance of their interactions with other food components. Their effects on tumor growth depend upon background levels of n-6 polyunsaturated fatty acids and antioxidants, and this could account for previously inconsistent results in experimental carcinogenesis. Recognition of the role of lipoperoxidation in the anti-tumor effects of polyunsaturated fatty acids, which is apparent in a variety of in-vitro or in-vivo systems, has been a major advance in the field. Consequently, n-3 polyunsaturated fatty acids appear to be excellent substrates for lipid peroxidation in situations where an oxidative stress is involved, such as in the action of several cytotoxic agents in the treatment of cancer.