Obsessive-compulsive disorder is a severe condition. Unfortunately, current medication and behavior therapies fail to adequately benefit some patients most severely affected. Advancing knowledge of brain circuit involvement has potential treatment implications. The neurosurgical techniques most often used in the United States for crippling, treatment-refractory obsessive-compulsive disorder are reviewed in the context of anatomic models of the illness. The use of transcranial magnetic stimulation to probe possible neuroanatomic and neurophysiologic substrates of this disorder is considered, and how the knowledge gained from such studies might advance treatment is presented.