Obsessive-compulsive disorder (OCD) affects approximately 2-3% of the adult population and is considered a debilitating and costly disorder, with associated impairments spanning the social, occupational, and familial domains. Although effective treatments of OCD exist, many individuals who suffer from OCD go undiagnosed or misdiagnosed, preventing them from obtaining appropriate treatment. As a result, making improvements to the assessment and diagnosis of OCD remains an important area of focus for research and clinical practice. This paper provides a critical review of instruments used in the assessment and diagnosis of OCD as well as a review of adjunctive measures used to assess associated symptoms. Types of instruments reviewed include diagnostic interviews, self-report questionnaires, family-report questionnaires, and clinician-administered inventories. Discussion of each instrument includes information regarding the pragmatics of administration and the psychometric properties of each instrument, as well as an evaluation of each instrument's strengths and weaknesses. We conclude by providing a synthesis of the literature and highlighting directions for future research.