AbstractHormone receptors are proteins located on the cell membrane or in the cell cytoplasm. Their function is to recognize and bind the respective hormone; the biological action of the hormone originates from the hormone‐receptor complex. Specific receptors have been found for all known hormones. Monitoring the equilibration between hormone, receptor, and hormone‐receptor complex (“determination of receptor binding”) permits quantitative determinations of hormones, allows the characterization of endocrinal disturbances, and contributes to the elucidation of structure‐affinity relationships.