A substantial body of scientific evidence suggests that obsessive-compulsive behavior occurs in a large percentage of patients with TS. Reliable estimates suggest that nearly 50% of patients with TS have some degree of obsessive-compulsive features. Most patients with TS have only mild OCB and thus would not meet the DSM-IV diagnostic criteria for OCD. Therefore, OCB is perhaps a more appropriate characterization of this behavioral phenomenon that occurs in TS. OCB in TS appears similar to the spectrum of the tic disorder in terms of its onset, severity, and course. As with tics, OCB is typically mild and not always substantially disabling. Although clinical features between TS + OCB and primary OCD overlap considerably, patients with TS + OCB appear to experience different types of obsessive thoughts and compulsive rituals. Compelling genetic evidence suggests that OCB may be an alternative expression of the TS phenotype, which may selectively affect female gene carriers. Identifying the TS gene in the future will substantially broaden our knowledge of this intriguing neurobehavioral disorder. Finally, neurobiologic evidence points to similar anatomic and chemical substrates in the pathogenesis of TS and OCD, suggesting that these two disorders share a common pathophysiology. The clinical evaluation of patients with TS and their families should always include an assessment for OCB. Self-rated inventories of OCD such as the LOI, LOI-CV, and MOCI are useful screening scales. A more structured interview using the Y-BOCS (CY-BOCS) is useful for determining the degree and severity of OCB in TS as well as the response to therapy. Clinicians should keep in mind that OCB may be the most disabling feature of TS and may require treatment. Pharmacologic agents, such as SSRIs, and traditional behavioral therapy are proven effective treatments for OCB, which can substantially reduce the full effect of TS on patients and their families.