This chapter reviews popular clinician‐administered and self‐report instruments for assessing severity of a range of obsessive‐compulsive and related disorders (OCRDs), including OCD, hoarding disorder (HD), trichotillomania (TTM), excoriation disorder (ED), and body dysmorphic disorder (BDD). It focuses on the assessment of DSM‐5 OCRDs, and includes disorders problems that are often comorbid with OCRDs or that have been previously grouped with OCRDs in the literature on these disorders, such as tic disorders (e.g., Tourette's disorder), hypochondriasis and health anxiety, and perfectionism. The chapter aims to discuss relevant assessment tools and to provide recommendations for clinicians by summarizing the purpose, psychometrics, strengths, and weaknesses for the most commonly used assessment measures. It is important for assessment to include a range of strategies, including a thorough clinical interview (perhaps with multiple informants) and behavioral methods, such as diaries, behavioral approach tests, and direct measures of target behaviors (e.g., collecting pulled hairs, in the case of trichotillomania).