The past decade has seen tremendous strides in the knowledge about the cause, epidemiology, and treatment of OCD. Research on clinical characteristics of the disorder have focused on several areas, including identification of subtypes, the role of insight, and patterns of comorbidity. Several studies looking at course of illness in OCD have found that, for adults with this disorder, the course is usually chronic, but increasing evidence shows that a subtype of OCD characterized by an episodic course may exist, and research is focusing on delineating that subtype more specifically. Another hypothesized subtype, which may be related to rheumatic fever, involves patients with both OCD and chronic tic disorders. Certain obsessions and compulsions are more common in patients with these two disorders; together with the familial transmission and treatment data, this suggests that these patients may represent a meaningful subtype. Another area of focus over the past 10 years has been the role of insight. Increasing evidence shows that a range of insight exists in patients with OCD. Whether patients with poor insight have a different treatment response or different course than do patients with better insight remains to be seen. Finally, comorbidity between OCD and schizophrenia has been an area of interest. Emerging evidence shows that obsessions and compulsions are more common in patients with schizophrenia than was previously thought. The effect of obsessions and compulsions on schizophrenia in terms of treatment response and course is being investigated. Despite tremendous advances in treatment of this potentially debilitating disorder, a significant percentage of patients do not respond to standard treatment. Continued research to identify meaningful subtypes in OCD is necessary to unravel important questions concerning cause and to develop specific treatment strategies for refractory patients.