A comprehensive, state-of-the-art review of our current understanding of the molecular and structural biology of 5-HT receptors and their potential use for drug discovery. The authors describe the anatomical, cellular, and subcellular distribution of 5-HT receptors and demonstrate a powerful approach to elucidating their physiological role using knockout mice in which the 5-HT receptors were deleted. They also review our understanding of the physiological role(s) of 5-HT receptors based mainly on studies performed in genetically engineered mice. Highlights include discussions of the behavioral phenotypes of 5-HT receptor knockout animals, the molecular biology and pharmacology of 5-HT receptors, and insights into the complexity of 5-HT receptor signal transduction.