Obsessive-compulsive disorder (OCD) is a severe and debilitating psychiatric illness characterized by intrusive unwanted thoughts (obsessions) and repetitive or ritualistic actions intended to reduce anxiety (compulsions). OCD is common with lifetime prevalence estimated at 2.5%, equally affecting both genders and all ethnic groups. It is also typically chronic in nature, resulting in substantial impact on quality of life. Functioning can be markedly impaired; it has been estimated that 20% of individuals with OCD should be considered severely affected. Although the pathophysiology is unclear, hyperactivity in the prefrontal orbital cortex has been consistently demonstrated, and normalizes with treatment. A decrease in cortical inhibition has been implicated in OCD utilizing a variety of methods, including paired-pulse TMS.