Obsessive-compulsive disorder is a severe and disabling clinical condition that usually arises in late adolescence or early adulthood and, if left untreated, has a chronic course. Whether this disorder should be classified as an anxiety disorder or in a group of putative obsessive-compulsive-related disorders is still a matter of debate. Biological models of obsessive-compulsive disorder propose anomalies in the serotonin pathway and dysfunctional circuits in the orbito-striatal area and dorsolateral prefrontal cortex. Support for these models is mixed and they do not account for the symptomatic heterogeneity of the disorder. The cognitive-behavioural model of obsessive-compulsive disorder, which has some empirical support but does not fully explain the disorder, emphasises the importance of dysfunctional beliefs in individuals affected. Both biological and cognitive models have led to empirical treatments for the disorder-ie, serotonin-reuptake inhibitors and various forms of cognitive-behavioural therapy. New developments in the treatment of obsessive-compulsive disorder involve medications that work in conjuction with cognitive-behavioural therapy, the most promising of which is D-cycloserine.