Technical Abstract: This elucidation of the role of selenium in health, which has occurred only over the span of the last five decades, is evaluated. This has involved selenium being first recognized as a toxicant, followed by it being shown to be an essential nutrient. The elucidation of the role of selenium in nutrition has led to fundamental discoveries in metabolic biochemistry, virology and public health. These have involved, respectively, three important discoveries: i) a previously unrecognized amino acid, selenocysteine, and its unique mode of co-translational incorporation into specific selenoproteins; ii) that selenium deprivation can lead to genomic destabilization of RNA viruses; and iii) that selenium can effect reductions in cancer risk. The evolution of understanding of the health relevance of selenium has been unlike that of other nutrients, all of which were recognized due to the fatal outcomes of their deficiencies. Instead, the health consequences of Se deprivation are largely sub-clinical in nature, requiring other precipitating factors (e.g., vitamin E deficiency, viral exposure, etc.) to reveal the effects of compromised Se-enzymes and/or essential Se-metabolites. Also unlike other nutrients, supranutritional levels of selenium have important anti-carcinogenic functions.