Obsessive compulsive disorders (OCD) are a nosographic entity. Their biological rating in serotonergic pathways and the efficacy of serotonergic antidepressants allows for developing a clinical and biological models of OCD. J. Guyotat, one of the first in 1959 to observe the favorable effects of antidepressants on OCD, presents their history. Epidemiological surveys conducted since 1980 have shown that the prevalence of OCD was underestimated until then. The prevalence is 2 to 3% in the adult population, with more women affected. The disorder develops early in childhood and adolescence. Loss of time is an important criteria for OCD but, according to M. Bourgeois, who reviewed the symptoms precisely, this does not warrant identifying a separate "primary obsessive slowness" syndrome. According to M. Bouvard, the prognosis of the disorder, in contrast to that for rituals observed in children between 3 and 5 years of age, is poor, with a risk of chronicity and social disturbances. The prevalence of OCD in children and adolescents is 0.8% and remains stable. The comorbidity, in particular with tics, is discussed. The favorable effects of fluoxetine are reported. J.M. Chignon reviews the concept of comorbidity, developed in internal medicine, and explains that it could be rigorously applied to psychiatry only starting with the DSM III-R. The comorbidity of OCD with other psychiatric diseases is highly variable: it is reviewed for personality disorders (0 to 55%), schizophrenia (4%), substance abuse (10%) and especially depression: one third of patients with OCD will develop a major depressive episode. Based on a clinical case report, M. Faruch leads us from symptoms to behavior therapy. The symptom must be considered for itself, whether it is part or not of the obsessive neurosis. It is legitimate to use antidepressants in combination with behavior therapy.