This chapter discusses obsessive-compulsive and related disorders. Obsessions are recurrent and persistent thoughts, urges, or images that are intrusive and unwanted. Compulsions are defined as repetitive behaviors or mental acts that an individual feels compelled to perform in response to obsessions or rules. People with obsessive-compulsive disorder (OCD) and related disorders often have other comorbid mental disorders. OCD and related disorders are closely related to each other and to anxiety disorders. Differentiating between subclinical symptoms and an actual disorder requires clinical judgment and consideration of the person’s level of distress and functional impairment. The following are the types of OCD and related disorders: OCD, body dysmorphic disorder, hoarding disorder, trichotillomania, excoriation disorder, substance- or medication-induced OCD and related disorder, OCD and related disorder due to another medical condition, other specified OCD and related disorder, and unspecified OCD and related disorder.