A complete description of a receptor-drug interaction includes an analysis of the binding step and an analysis of the coupling between the binding and the biological response. Both steps of the analysis can be very complicated. In the case of the binding step, the receptor molecule may exist in more than one conformational state each of which exhibit different affinities for the drug. A complete description of the binding mechanism would include a characterization of the affinity of the drug for each conformational state as well as a description of the isomerization between conformational states. The coupling between the binding and the biological response is difficult to discuss in general terms because it is highly dependent upon the tissue under study. Nevertheless, a number of strategies have been developed that are useful in a variety of cases.