Obsessive–compulsive disorder (OCD) is a neuropsychiatric illness that often develops in childhood, affects 1%–2% of the population, and causes significant impairment across the lifespan. The first step in identifying and treating OCD is a thorough evidence-based assessment. This paper reviews the administration pragmatics, psychometric properties, and limitations of commonly used assessment measures for adults and youths with OCD. This includes diagnostic interviews, clinician-administered symptom severity scales, self-report measures, and parent/child measures. Additionally, adjunctive measures that assess important related factors (ie, impairment, family accommodation, and insight) are also discussed. This paper concludes with recommendations for an evidence-based assessment based on individualized assessment goals that include generating an OCD diagnosis, determining symptom severity, and monitoring treatment progress.