The UCLA Anxiety Disorders Program is currently recruiting adults with Obsessive Compulsive Disorder in a free Cognitive Behavioral Therapy (CBT) treatment research study at UCLA. The purpose of this project is to determine what, if any, changes in brain chemistry and brain structure take place in adults as a result of Cognitive-Behavioral Therapy, a form of non-drug psychotherapy, for OCD. We will measure amounts and distribution of the neurotransmitter chemical glutamate in the brain using MRSI (Magnetic Resonance Spectroscopic Imaging) scans before and after 4 weeks of CBT.