The regulation of metabolic processes, cell growth and differentiation is achieved by an interaction between hormones and specific cellular binding sites termed "receptors". These may be located on the cellular surface, in the cytoplasm or in the nucleus. Recently the structure of several membrane receptors in mammalian cells have been elucidated. These consist of peptide chains possessing multiple functional "domains". We describe in details the extracellular, transmembrane and intracellular "domains". In recent years antibodies to many receptors and analogues of some peptide hormones have become available; these can be used in clinical practice to study receptors, their localization and, in some cases, to block their function.