Selenium is an essential dietary component for animals including humans, and there is increasing evidence for the efficacy of certain forms of selenium as cancer-chemopreventive compounds. In addition, selenium appears to have a protective effect at various stages of carcinogenesis including both the early and later stages of cancer progression. Mechanisms for selenium-anticancer action are not fully understood; however, several have been proposed: antioxidant protection, enhanced carcinogen detoxification, enhanced immune surveillance, modulation of cell proliferation (cell cycle and apoptosis), inhibition of tumor cell invasion and inhibition of angiogenesis. Research has shown that the effectiveness of selenium compounds as chemopreventive agents in vivo correlates with their abilities to affect the regulation of the cell cycle, to stimulate apoptosis and to inhibit tumor cell migration and invasion in vitro. This article reviews the status of knowledge concerning selenium metabolism and its anticancer effects with particular reference to the modulation of cell proliferation and the inhibition of tumor cell invasion.