The identification of multiple receptor subtypes for 5-hydroxytryptamine (5-HT) made by using radioligand binding techniques proliferated at a brisk rate in the 1980s. The application of molecular biological techniques to 5-HT receptor studies is likely to lead to an expansion rather than a reduction in the number of distinct 5-HT receptor subtypes. Although the current status of 5-HT receptor pharmacology may appear to be overwhelmingly confusing to most investigators, the evolving data suggest that 5-HT receptor subtypes can be categorized into three major families. Each family consists of multiple receptor subtypes that share similarities in their molecular biological, pharmacological, biochemical, and/or physiological properties. This review provides a summary of recent data as well as a framework for the classification of 5-HT receptor subtypes.