This study examines the efficacy of the medication D-Cycloserine in the reduction of obsessions and compulsions in individuals with obsessive-compulsive disorder (OCD). All study participants will receive Exposure and Response Prevention, a common form of behavior therapy for individuals with OCD. Half (50%) of the participants will be randomly assigned to the placebo condition and the other half (50%) will be randomly assigned to receive D-Cycloserine. Because all participants will receive 10 sessions of behavior therapy, all participants are expected to improve from this treatment.