This volume attempts to assemble the current information on the ligand (vitamin) – receptor (binding protein) interaction as it applies to each of the vitamins, from the site of enteric absorption to the site of vitamin function. For the absorption and transport of most vitamins, specific proteins, which have a recognition factor incorporated into them, are involved. At the plasma membrane of the cell, a second recognition factor has to function before internalization of the vitamin. Intracellular vitamin-binding proteins and their interaction with cellular systems resulting in the biological action of the vitamins provides the last step in the successive stages of communication. Thus, we have the concept of a ligand, an extraorganismic molecule, reacting with a specific protein entity, the receptor, facilitating absorption and transport of the ligand to its site of action with the resultant sequence of physiological events to which we refer as ‘vitamin function’. The term ‘receptor’, in the context of vitamins, is used to refer to proteins with a recognition function and not to receptors in the traditional pharmacological sense. It includes proteins which bind to the vitamin ligand and function as extracellular or intracellular transporters as well as the smaller number of vitamin-binding nuclear proteins. In the latter case, vitamins might act as small molecular modulators of protein-DNA interaction, a situation analogous to that of some hormones. The designation ‘receptors’ for these categories of vitamin-binding proteins appears to be appropriate as it connotes a function, more than ligand binding.