A fundamental mistake in receptor theory has led to an enduring misunderstanding of how to estimate the affinity and efficacy of an agonist. These properties are inextricably linked and cannot be easily separated in any case where the binding of a ligand induces a conformation change in its receptor. Consequently, binding curves and concentration–response relationships for receptor agonists have no straightforward interpretation. This problem—the affinity–efficacy problem—remains overlooked and misunderstood despite it being recognized in 1987. To avoid the further propagation of this misunderstanding, we propose in this review that the affinity–efficacy problem should be included in the core curricula for pharmacology undergraduates proposed by the British Pharmacological Society and the International Union of Basic and Clinical Pharmacology (IUPHAR).