Selenium (Se), an essential trace element, has also been identified as an anticarcinogenic agent, with supporting evidence from epidemiological studies, clinical intervention trials, preclinical intervention studies (animal cancer models) and cell culture studies. Natural organic and inorganic sources of Se as well as synthetic organoselenium compounds have been shown to be effective; safety and efficacy factors favour the organic forms. Intakes that are several fold that purported to meet nutritional requirements (adult recommend dietary allowance--55 μg Se/day) are associated with reductions in cancer risk, but are not currently met by most diets, unless Se-rich foods are included. Further clinical studies and development of tools for speciating Se in foods will enable progress to be made in determining desirable Se forms and foods with respect to providing safe and effective ways of reducing cancer risk.