Recent epidemiological studies have demonstrated that selenium may be an effective chemopreventive and anticancer agent with a broad spectrum against several human cancer cells (prostate, colon, bladder, lung, liver, ovarian, leukemia). A wide range of potential mechanisms have been proposed for the antitumorigenic effects of selenium and these include antiandrogen activity, growth inhibitory effects by regulation of p53 and antioxidant function, and through DNA damage. However, apoptosis is one of the most plausible mechanisms for the anticancer activity. The regulating mechanisms of apoptosis are extremely complex and for selenium compounds they mainly involve a mitochondrial pathway, protein kinases, tumor necrosis factor, activation of caspases and reactive oxygen species. The aim of this review is to summarize the current knowledge about more than twenty eight selenium-containing molecules and to discuss the implications for apoptosis and the impact in cancer therapy.