Significant advances in the analysis of 5-hydroxytryptamine (5-HT) receptor subtypes occurred in the 1980s. Although the current status of 5-HT receptor pharmacology may appear confusing to many 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. These receptor subtypes are present throughout the nervous system as well as in the periphery. A simple framework for the classification of 5-HT receptor subtypes is provided in the present review.