The experimental design covers 2 phases. Phase I is a 2 (site) x 2 (SER or pill PBO) x 3 (CBT, ES or non- psychosocial treatment) x 5 (repeated measures) factorial 12-week comparison of SER, CBT, COMB and the control conditions. In Phase II, responders advance to a 16-week discontinuation study to assess treatment durability. The primary outcome measure is the Yale-Brown Obsessive-Compulsive Scale. Assessments blind to treatment status take place at Week 0 (pretreatment); Weeks 1, 4, 8, 12 (Phase I treatment); and Weeks 16, 20, 24 and 28 (Phase II discontinuation). Besides addressing comparative efficacy and durability of the specified treatments, the investigators also examine time-action effects, differential effects on specific aspects of OCD, including functional impairment, and predictors of response to treatment.