Obsessive-compulsive disorder (OCD) is characterized by repetitive thoughts and behaviours that are experienced as unwanted. Family and twin studies have demonstrated that OCD is a multifactorial familial condition that involves both polygenic and environmental risk factors. Neuroimaging studies have implicated the cortico-striato-thalamo-cortical circuit in the pathophysiology of the disorder, which is supported by the observation of specific neuropsychological impairments in patients with OCD, mainly in executive functions. Genetic studies indicate that genes affecting the serotonergic, dopaminergic and glutamatergic systems, and the interaction between them, play a crucial part in the functioning of this circuit. Environmental factors such as adverse perinatal events, psychological trauma and neurological trauma may modify the expression of risk genes and, hence, trigger the manifestation of obsessive-compulsive behaviours.