Vitamins are essential organic nutrients required by animals in only tiny amounts. As animals have evolved they have come to rely on their diet to supply the metabolites now recognised as vitamins rather than synthesise them for themselves. Deficiencies cause specific deficiency syndromes. Vitamins are differentiated from various other dietary components: essential fatty acids and indispensable amino acids are required in larger amounts, trace metals are not organic, hormones are synthesised by the body as required rather than obtained from the diet and many other substances, while beneficial in the treatment of a particular illness do not invoke that illness when omitted from the diet. The history of vitamin discovery has led to often confusing nomenclature (B₁, C, etc.). Vitamins are classified as either water soluble, i.e. thiamin, riboflavin, pyridoxine, pyridoxol, niacin, cobalamin, folic acid, biotin, pantothenic acid and ascorbic acid or fat soluble, i.e. retinol, cholecalciferol, vitamin E and vitamin K. The distribution of vitamins in foods, their physiological roles (including function at the biochemical level), and stability in food processing etc. are now well understood. The chapter concludes with a list of specialist books and review articles for further study.