The anxiety disorders are conventionally subdivided into: generalised anxiety disorder (GAD), panic disorder (PD), social phobia (social anxiety disorder (SAD)), posttraumatic stress disorder (PTSD) and obsessive-compulsive disorder (OCD). The current edition of the Diagnostic and Statistical Manual of Mental Disorders, the (DSM)-IV-TR lists 12 anxiety disorders, with broadly similar categories listed in the International Classification of Impairments, Disabilities and Handicaps, 10th revision (ICD-10). Neurochemistry and brain imaging studies have influenced the development of theoretical models of GAD. Current neuroanatomical models of panic disorder draw on the understanding of the functional neuroanatomy of fear conditioning in animals. SAD is second only to specific phobia as the most common anxiety disorder in population-based community studies; onset is in the mid-teenage years and extends into the middle of the third decade. There are a wide range of medical conditions that can cause and perpetuate symptoms of anxiety.