Chemical elements such as selenium, fluoride, iron, calcium and magnesium are essential to the human being, although some are toxic when absorbed in high doses. In this paper, the risks associated with insufficient and excessive intake of selenium in the diet are reviewed, focusing on drinking water. Two different approaches are used to derive recommended nutrient intakes (RNI) for adequate nutritional status and guideline values to prevent excessive exposure. The former is based on the daily intake which meets the nutrient requirements of 97.5% of the population. The latter is a value derivation based on an assumed daily per capita consumption at the individual level, a conservative approach used where there is any uncertainty and is related to a negligible risk to health at population level across life stages. There is an increasing need to develop a conceptual framework bringing together aspects of toxicity and essentiality especially for elements apparently exhibiting narrow or overlapping ranges between essentiality and toxicity and to provide guidance on the nature and severity of risks in order to better protect human. While there are a number of frameworks available, these generally only consider food. There is a need to include water, which can be a significant source in some circumstances.