Obsessive-compulsive disorder (OCD) affects approximately 2-3% of the adult population. OCD is associated with impairments in social and occupational functioning. Although there are effective treatments for OCD, many individuals who suffer from OCD go undiagnosed and untreated. Thus, improvements in the assessment and diagnosis of OCD are needed. This paper provides a review of the instruments used to assess and diagnose patients with OCD. We reviewed clinician-administered inventories and self-report questionnaires. The characteristics, strengths, weaknesses, reliability and validity of each instrument are discussed. We also reviewed the psychometric properties of the Korean version of each instrument.