The clinical efficacy of serotonin reuptake inhibitors such as clomipramine in the treatment of obsessive compulsive disorder(OCD) has fueled interest in the neurobiological basis of this illness. OCD is responsive exclucively to potent serotonin reuptake inhibitors clomipramine, fluoxetine, fluvoxamine, sertraline, and paroxetine and this point forms the important evidence supporting a cental role for serotonin in the pathogenesis of the disorder. Other serotonergic medications such as lithium, buspirone, trazodone, or fenfluramine may be useful as adjuvant treatments in treatmentrefractory OCD and adjuvant antipsychotics are useful in tic disorders, personality disorders, and psychotic disorders. This paper reviews results of treatment studies, investigations of biological markers, and neuroendocrine challenges and implications for the role of serotonin in pathophysiology and treatment of OCD.