PIP: This article reviews the literature on hormone receptors and presents information on the following broad topics: 1) steroid hormone receptors (vs. cytoplasmic); 2) thyroid hormone receptors; 3) catecholamine and peptide hormone receptors; 4) internalization of polypeptide and catecholamine hormone-receptor complexes; 5) affinity (as the basis for discrimination of hormonal signals); 6) binding curves and Scatchard plots; 7) nonspecific and nonreceptor binding; 8) curvilinear Scatchard plots, indicating multiple sites and negative cooperativity; 9) definition of class of hormone action and hormonal occupancy of several classes of receptors; 10) receptor-response relation (close coupling and spare receptors); 11) regulation of receptors (a mechanism important for control of cellular sensitivity to hormones); 12) hormone antagonists and partial agonists and antagonists; 13) receptors and the clinical evaluation of hormone and drug levels; and 14) receptor-related diseases. Throughout the literature, identification of hormone-binding sites as receptors has relied mainly on correlations between hormone binding and biologic effects, on genetic and other studies where loss of (or changes in) receptors are associated with altered hormone responsiveness, and on the finding that receptor binding by anti-receptor antibodies or antagonists attentuates or abolishes the hormone responses.