Obsessive‐compulsive disorder (OCD) is characterized by the presence of unwanted, senseless, intrusive and distressing thoughts, urges and images (obsessions) and/or repetitive behaviors (compulsions). While classified as an anxiety disorder in DSM‐IV, OCD is given a separate chapter here as the clinical phenomenology, drug treatment response, family studies, and brain imaging data support a separate neurobiological classification for this disorder and a distinct neuropharmacologic profile as well. Our current understanding of its underlying neurobiology as well as the state of treatment for OCD is provided through a selective review of OCD‐related (1) brain imaging studies, (2) genetic studies, (3) animal models, (4) psychopharmacology, including immunomodulatory treatment, and (5) experimental non‐pharmacological treatments such as neurosurgery, transcranial magnetic stimulation, and deep brain stimulation.