This chapter focuses on cognitive models (also known as cognitive-behavioral models) of obsessive-compulsive disorder (OCD) and related disorders. The models posit that appraisals, dysfunctional beliefs, and maladaptive behaviors play important roles in the etiology and maintenance of obsessive-compulsive and related disorders (OCRDs). The chapter begins with an historical perspective in which the antecedents of the models are described. Contemporary cognitive models of OCD are described, and their empirical support is reviewed. This is followed by a review of cognitive models of four OCRDs: hoarding disorder, skin-picking disorder (excoriation), body dysmorphic disorder, and hair-pulling disorder (trichotillomania). Conceptual strengths and weaknesses of OCD and OCD-related cognitive models are identified, areas for improvement are identified, and potentially fruitful directions for future research are proposed.