Obsessive–compulsive disorder (OCD) is a neuropsychiatric disorder that affects approximately 2% of the population world-wide and has strong biological underpinnings and a well-defined neurocircuitry. Neuroimaging studies have implicated the ventromedial prefrontal cortex (VMPFC), the anterior cingulate cortex (ACC), the orbitofrontal cortex (OFC), the striatum (particularly the caudate nucleus), and the thalamus, as being involved in the pathophysiology of OCD. These brain regions are interconnected in multiple recurrent loops, making up the cortico-striatal-thalamic-cortical circuit, and are thought to be involved in action selection, performance monitoring, and goal directed behaviors. Hyperactivity of this circuit has been demonstrated in individuals with OCD, both at rest and following symptom provocation. It has been proposed that, in this disorder, such cortico-striatal hyperactivity leads to a persistently high error signal, ultimately resulting in the psychopathology characteristic of OCD. In this model, the brain’s error monitoring system compares intended responses to actual responses, and generates an error signal when a conflict is detected. It has been suggested that this error signal is amplified in individuals with OCD, leading to the feeling that something is “out of line” or that an action was not completed correctly according to a set of internal unattainable rules, triggering repetitive, compensatory behaviors.