One of the ongoing problems in biological psychiatry lies in relating clinical laboratory findings to specific symptoms or behaviour patterns. In recent years it has become clear that serotonin (5-HT) is involved in a wide range of psychiatric disorders and in quite specific behaviour patterns, mainly characterized by a poor control of impulses. Psychopharmacotherapy with substances able to enhance the metabolism of serotonin (mainly antidepressants) is able to clinically improve what appear to be very dissimilar conditions. Moreover, antidepressants with serotonergic activity are the only class of drugs able to relieve the symptoms of obsessive compulsive disorder (OCD), which are characterized by a high control of impulses or, more precisely, by a dysfunctional, non-adaptive control of impulses. Recent neurochemical data suggest that a hypersensitivity to 5-HT exists in this condition. Therefore, the hypothesis that serotonin is important for efficient control of impulses is a key to the interpretation of many laboratory findings and to the penetration of the complex field of biological substrates of psychopathology.