Over the past few years, the concept that the activation of G protein-coupled receptors and transmitter-gated ion channels depends on a conformational change has received increasingly widespread acceptance. As a result, these two structurally distinct families of receptors can now be considered to obey a similar two-state mechanism. However, traditional receptor theory has largely overlooked this concept. In this article, Paul Leff explains and illustrates the predictions of the two-state model of receptor activation and discusses its impact on the analysis and interpretation of agonist-receptor interactions.