The authors review current neurobiological models of obsessive-compulsive disorder (OCD). The rationale for anatomically based models is outlined and selected brain regions of interest are discussed. The authors conclude that there is abundant evidence to implicate multiple brain regions as sites of abnormality in OCD. A review of neuropharmacological concepts related to the serotonergic (5-HT) hypothesis of OCD is also presented. The authors conclude that anti-obsessional agents may well have therapeutic effects via the 5-HT system, yet there is little unequivocal evidence to suggest an underlying abnormality in the 5-HT system. A comprehensive neurobiological model of OCD must acknowledge the interrelationship between multiple transmitter systems and integrate anatomical with neuropharmacological concepts.