Evidence-based assessment is a cornerstone for evidence-based treatment. This chapter provides an introduction to commonly used rating scales of OCD symptom severity for an evidence-based assessment. This includes clinician-administered measures, self-report, and parent-and-child report rating scales. Given the importance of OCD symptom dimensions, this chapter also presents measures to assess specific OCD symptom dimensions. The chapter concludes with considerations for selecting rating scales as part of an evidence-based assessment for children and adults with OCD.