Quantitative review of the controlled treatment outcome literature for obsessive-compulsive disorder (OCD) showed that exposure with response prevention was highly effective in reducing OCD symptoms. Cognitive approaches were also found to be at least as effective as exposure procedures. It appears that both cognitive and exposure interventions involve some overlapping procedures and capitalize on similar mechanisms of change. Serotonergic medication, particularly clomipramine, also substantially reduced OCD symptoms. However, clomipramine may not be particularly superior to other serotonergic medication. The relationship between side effects and effect size in medication trials was explored.