Advances in obsessive-compulsive disorder (OCD) research have led to increased attention to a range of disorders with possibly overlapping phenomenological and neurobiological features; the so-called OCD spectrum disorders. This article briefly reviews neurobiological data relevant to the construction of an OCD spectrum, including neurochemical, neuroanatomic, genetic, neuroimmunology, and animal studies. OCD and related disorders may be heterogenous conditions, and the neurobiology of many putative OCD spectrum disorders has not been well studied. Nevertheless, a gradual accumulation of neurobiological data has provided a number of exciting, and partially overlapping, approaches to an hypothesized OCD spectrum.