The physiologic role of adrenocorticotropic hormone (ACTH) and its part in the pituitary—adrenal axis is one of the most intensively studied systems in endocrinology. ACTH was one of the first hormones that was found to stimulate cAMP production by the adrenal gland (1) , and the notion that this effect was mediated via a specific cell surface receptor was confirmed by the elegant studies of Lefkowitz and colleagues (2) in work that set a standard for receptor characterization. Nevertheless, progress on the understanding of the ACTH receptor has been relatively slow. It is now clear that the MC2-R is synonymous with the ACTH receptor, and both terms are used in this chapter. In general, the term ACTH receptor is used to describe the functional entity for example, ligand binding to adrenal cells, whereas the term MC2-R is used to describe aspects that can clearly be related to this gene. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.