Epidemiological data provide evidence that it is possible to prevent cancer and other chronic diseases, some of which share common pathogenetic mechanisms, such as DNA damage, oxidative stress, and chronic inflammation. An obvious approach is avoidance of exposure to recognized risk factors. As complementary strategies, it is possible to render the organism more resistant to mutagens/carcinogens and/or to inhibit progression of the disease by administering chemopreventive agents. In a primary prevention setting, addressed to apparently healthy individuals, it is possible to inhibit mutation and cancer initiation by triggering protective mechanisms either in the extracellular environment or inside cells, e.g., by modifying transmembrane transport, modulating metabolism, blocking reactive species, inhibiting cell replication, maintaining DNA structure, modulating DNA metabolism and repair, and controlling gene expression. Tumor promotion can be counteracted by inhibiting genotoxic effects, favoring antioxidant and anti-inflammatory activity, inhibiting proteases and cell proliferation, inducing cell differentiation, modulating apoptosis and signal transduction pathways, and protecting intercellular communications. In a secondary prevention setting, when a premalignant lesion has been detected, it is possible to inhibit tumor progression via the same mechanisms, and in addition by affecting the hormonal status and the immune system in various ways, and by inhibiting tumor angiogenesis. Although tertiary prevention, addressed to cancer patients after therapy, is outside the classical definition of chemoprevention, it exploits similar mechanisms. It is also possible to affect cell-adhesion molecules, to activate antimetastasis genes, and to inhibit proteases involved in basement membrane degradation.