This chapter identifies clinically meaningful subtypes of obsessive-compulsive disorder (OCD) including discrete categories based on age of onset and comorbidity with related disorders, and then describes efforts to understand obsessive-compulsive (OC) symptoms as dimensions. Both categorical and dimensional approaches have been applied in research on refining the phenotype of OCD. A strategy that has been successfully applied to other heterogeneous and complex psychiatric disorders is to identify intermediate phenotypes that are more closely related to neurobiological mechanisms than phenotypes. Modern neuroimaging techniques have allowed abnormalities in white-matter brain tissue to be investigated as potential OCD endophenotypes. Subtypes of OCD have been proposed, based on early age of onset, tic comorbidity, and obsessive-compulsive personality disorder (OCPD) comorbidity. Another approach to deconstructing the heterogeneity of OCD is the identification of vulnerability markers or endophenotypes. Integrating phenotypical and endophenotypical approaches may be fruitful in advancing the field.