According to DSM-IV criteria, obsessive compulsive disorder (OCD) is an anxiety disorder that is characterized by recurrent, intrusive images or thoughts and/or stereotyped, repetitive behaviors that are associated with marked distress, anxiety, or psychosocial impairment. The differential diagnosis of OCD can be quite difficult since OCD symptomatology can occur as either primary or secondary phenomena. Comorbid depression or personality disorder is not uncommon in patients with primary OCD. Other comorbid conditions that occur with OCD can be divided into three major groups based on core features: (1) disorders of altered risk assessment; (2) incompleteness/habit-spectrum disorders; and (3) psychotic spectrum disorders. Such a categorization of core dimensions and comorbid conditions may prove useful in identifying distinct OCD subtypes that share underlying neurobiological or treatment response characteristics.