The demonstrated efficacy in anxiety disorders of drugs such as buspirone or fluoxetine has emphasized the importance of 5-hydroxytryptamine (5-HT or serotonin). Buspirone is a selective agonist at a subtype of serotonin receptor termed 5-HT1A, whereas fluoxetine is a selective inhibitor of the reuptake of 5-HT. At least 14 types of mammalian serotonin receptors have been isolated and classified into seven major families, using pharmacologic, transductional, and structural criteria. The subtypes of serotonin receptors are localized in different regions of the brain. Selective compounds for particular subtypes of serotonin receptors may yield selective pharmacologic effects. Since the latency between the initiation of treatment with an SSRI and the appearance of clinical effects may be due to the desensitization of presynaptic autoreceptors, the development of drugs to decrease latency is an active area of investigation. This article provides a brief overview of the physiology and pharmacology of serotonin systems so that the relationship between serotonin compounds and anxiety can be better understood.