When considering a diagnosis of obsessive-compulsive and/or related disorders, providers must assess many factors, including diverse presentation of symptoms, symptom overlap, comorbid conditions, and patient insight. The current chapter reviews these diagnostic complications and provides recommendations for accurately diagnosing obsessive-compulsive disorder (OCD) and related conditions. An overview of obsessive-compulsive symptom dimensions is discussed, along with descriptions of obsessive-compulsive-related disorders such as body dysmorphic disorder, hoarding disorder, trichotillomania, and excoriation disorder. Then, recommendations for formal and informal diagnostic interviewing and assessment are reviewed, which include common clinician-rated, self-report, and parent-report measures. Additionally, a case illustration is provided to show an example of an unstructured interview that is guided by structured assessment tools to help diagnose OCD. Finally, cultural variables to consider and critical information needed to make an accurate diagnosis are reviewed.