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Some of the main biochemical features of selenium have emerged only in the last five years, although it has been known to be an essential element for nearly 40 years. The investigations into selenoproteome gene expression and a better understanding of the selenocysteine synthetic pathway have undoubtedly provided the evidence that underpins the biochemical roles of the element. To date, 25 selenium-containing proteins have been identified in humans but the functions of a number of these have yet to be elucidated. The roles of the selenium-containing enyzmes (glutathione peroxidases, thioredoxin reductases and iodothyronine deiodinases) are well established, the first two being linked with antioxidant activity, and the latter involved with thyroid hormone metabolism. Recently, the interaction between sulphur, in the same periodic group and therefore chemically similar, and selenium has been investigated in a bid to understand the role of both elements in disease. There is renewed interest in the anticancer properties of selenium-containing compounds as evidence of their effectiveness in animal models has been demonstrated. Herein, selenium metabolism, gene expression, interaction with sulphur, and role in cancer are reviewed.