Demonstration of the efficacy of serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitors such as clomipramine in the treatment of obsessive compulsive disorder has fueled interest in the neurobiological basis of this illness. Results of treatment studies, investigations of biological markers, and pharmacologic challenges are reviewed and implications for a 5-HT theory of obsessive compulsive disorder discussed. While the nature of the dysregulation in serotonin transmission that may attend obsessive compulsive disorder has yet to be fully elucidated, evidence accumulates that 5-HT function in part modulates obsessive compulsive symptoms. Development of more specific probes and new brain imaging techniques will further enhance understanding of the pathophysiology of obsessive compulsive disorder.