Radioligand binding studies are relatively easy to perform, often very accurate, and can usually be interpreted according to a mechanistic model to yield fundamental chemical quantities, such as the affinity of a ligand for a receptor or the number of receptors in a tissue. For these reasons, binding studies are used in a very wide range 3of applications, from the simple detection of active ligands in a screening assay to detailed mechanistic studies of ligand binding, receptor activation, and receptor regulation. There are problems and pitfalls with any technique, and the power of binding studies to provide quantitative estimates of molecular parameters implies a special need for rigor in the interpretation of binding data. The basic protocols for binding studies are straightforward in general, but the specific details, and the quantities that can be estimated, depend crucially on the system under investigation.