Within the past decade the field of psychiatry has rediscovered the neuropsychiatric syndrome of obsessive–compulsive disorder (OCD). Although excellently described over 150 years ago, for many years OCD was thought to be rare, untreatable, and to arise from hidden psychodynamic conflicts. All of these earlier ideas now appear to be wrong. Occurring in approximately 2% of adults, OCD consists of recurrent intrusive thoughts (obsessions) or senseless repetitive actions (compulsions). Although the aetiology of OCD remains unclear, recent neuro-imaging studies implicate the basal ganglia and frontal cortex as crucial structures in the pathogenesis of OCD. Genetic studies demonstrate a clear genetic component to OCD and an interesting link with chronic motor tics and the Gilles de la Tourette Syndrome. Although a true cure for the disorder remains elusive, most OCD symptoms respond well to treatment with 5HT reuptake inhibitors. The phenomenology and aetiology of OCD will be reviewed, with particular emphasis placed on the proper pharmacological treatment of this sometimes crippling disorder.