Receptors, located on both the cell surface and within the cell, are the molecular targets through which drugs produce their beneficial effects in various disease states. Receptors were initially conceptualized at the beginning of the 20th century by the parallel efforts of Ehrlich and Langley. The concepts of the receptor and receptor theory, based on the Law of Mass Action, have undergone continuous refinement as they have been characterized in terms of their molecular structure, association with ancillary proteins (e.g., G proteins, arrestins, RAMPs), and functional characteristics in normal and diseased tissues. The concepts describing ligand interactions with receptors have also been refined from the simple binary concept of competitive agonists and antagonists to partial agonists, allosteric modulators and inverse agonists. Concepts such as receptor constitutive activity, internalization and dimerization add additional complexity to the role of receptors in tissue function and in precisely characterizing their role in homeostasis and disease.