Serotonin (5-hydroxytryptamine; 5-HT) receptors modulate a variety of physiological processes ranging from perception, cognition and emotion to vascular and smooth muscle contraction, platelet aggregation, gastrointestinal function and reproduction. Drugs that interact with 5-HT receptors effectively treat diseases as diverse as migraine headaches, depression and obesity. Here we present four structures of a prototypical serotonin receptor-the human 5-HT2B receptor-in complex with chemically and pharmacologically diverse drugs, including methysergide, methylergonovine, lisuride and LY266097. A detailed analysis of these structures complemented by comprehensive interrogation of signaling illuminated key structural determinants essential for activation. Additional structure-guided mutagenesis experiments revealed binding pocket residues that were essential for agonist-mediated biased signaling and β-arrestin2 translocation. Given the importance of 5-HT receptors for a large number of therapeutic indications, insights derived from these studies should accelerate the design of safer and more effective medications.