In this article, which is the first in a three-part series, the authors provide an overview of the current state of our knowledge of the phenomenology, etiology, and diagnosis of OCD. The DSM-IV criteria for OCD are presented and explicated. Disorders that are commonly comorbid with OCD (e.g., major depressive disorder, other anxiety dis-orders, Tourette's disorder) are described. The authors also discuss disorders such as body dysmorphic disorder that may be related to OCD and are often termed OCD spectrum disorders. OCD is likely to have multiple causes and the authors discuss behavioral, neuroanatomical, and neurochemical theories of OCD. Two treatments have demonstrated efficacy in OCD, cognitive-behavioral therapy and pharmacotherapy with serotonergic reuptake inhibitors, and the authors discuss how these treatments may work in light of what is known about the etiology of the disorder. The different subtypes of OCD that have been proposed are described along with their implications for treatment. The article concludes with a discussion of diagnosis that provides specific guidance for the clinician on how to assess a patient for possible OCD. The next two articles in this series will cover cognitive-behavioral and medication treatment in detail.