Obsessive-compulsive disorder (OCD) is an anxiety disorder that affects between 1% to 2% of individuals and causes considerable impairment and disability. Although > 50% of individuals experience symptom onset in childhood, symptoms can continue to develop throughout adulthood. Accurate and timely assessment of clinical presentation is critical to limit impairment and improve prognosis. Presently, there are 2 empirically supported treatments available for OCD in children and adults, namely cognitive-behavioral therapy and pharmacotherapy with serotonin reuptake inhibitors. This article provides an introduction to the phenomenology, etiology, and clinical course of OCD. Assessment practices used to evaluate symptom severity are described, and evidence-based treatment options are reviewed, with appropriate distinctions drawn between children and adults. Finally, recommendations for assessment and treatment practices for OCD are explicated.