Two treatments have demonstrated efficacy in OCD, exposure and response (ritual) prevention (EX/RP) and pharmacotherapy with serotonin reuptake inhibitors (SRIs). In this article, which is the third in a three-part series, the authors present an overview of the role of biological treatments for OCD. The evidence for the efficacy of the serotonin reuptake inhibitors (clomipramine and the five selective serotonin reuptake inhibitors "SSRIs" fluvoxamine, fluoxetine, paroxetine, sertraline, and citalopram) as monotherapy for OCD is reviewed. The authors also discuss the rationale for choosing among these agents for specific patients. Research on other types of medication monotherapies for OCD is also discussed. The authors then cover strategies for treatment-resistant OCD, including combining EX/RP and SRI medication treatment, combining clomipramine and an SSRI, use of augmenting medications, and use of intravenous clomipramine. Findings concerning the use of other somatic therapies for treatment-resistant OCD, including electroconvulsive therapy, neurosurgery, plasma exchange/IV immunoglobulin/maintenance antibiotics, and transcranial magnetic stimulation, are also reviewed. Finally, the authors discuss what is known about matching treatments to patients with certain specific symptom clusters, how long to continue maintenance medication treatment, and how to terminate treatment.