Almost anything that is swallowed in pill or potion form that is not a licensed medicine is, by default, legally classified as a dietary supplement. The present paper is an overview of supplement use and is intended to provide a logical framework for their discussion. Five major supplement categories are identified: essential micronutrients; other metabolites that have vitamin-like roles; natural oils; natural plant or animal extracts; antioxidants, which span the other categories. Supplement usage is also classified into broad categories. Examples of each supplement category and usage are briefly discussed. Some potential hazards of supplement use are also outlined; many substances in supplements are either not found in normal UK diets or consumed in much greater amounts than would be found in food. Many supplements are used for pharmaceutical purposes and sold as supplements to avoid the expense of acquiring a medicinal licence and to avoid the stricter quality-control regulations that apply to medicines. The use of supplements to ensure nutritional adequacy and as possible conditionally-essential nutrients is briefly discussed, as is their essentially pharmaceutical use for the prevention and alleviation of disease. There is critical discussion of whether the use of antioxidant supplements is justified or even if it is reasonable to promote a particular food on the basis of antioxidant content alone. Much of the research on supplements is reductionist, commercially sponsored or has other weaknesses; so, despite decades of use and research there is still uncertainty about their efficacy in many cases.