According to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), obsessive-compulsive and related disorders refer to a group of mental health disorders sharing psychopathological characteristics, comorbidity, and putative etiology. The DSM-5 chapter includes obsessive-compulsive disorder (OCD), hoarding disorder, body dysmorphic disorder, hair-pulling disorder, and skin-picking disorder. Individuals with tic disorders also show recurrent behaviors (motor or vocal) but are classified in a subchapter of Neurodevelopmental Disorders. OCD can be specified according to the DSM-5 as tic-related OCD if there is a present or lifetime comorbidity with a tic disorder. There is broad consensus that both biological and psychological factors contribute to the development and maintenance of OCD. The focus of this entry is on biological factors.