Technical Abstract: Selenium (Se) is an essential dietary component for mammals 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 potential mechanisms have been proposed. These include an 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. Over the years, research has shown that the effectiveness of Se-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 the molecular mechanisms of these cellular effects.