[Mental deficiency. Definition, epidemiology, etiology (author's transl)]

In this account, the author replaces this chapter within the field of pediatrics, showing the present importance of the problem. In spite of a definition which is sometimes imprecise, and epidemiological studies which are incomplete and difficult to draw up, the importance of this subject was obvious to those who drew up the 7th economic plan (1976--1980). Their estimation mentionned a total number of about 500,000 children, i.e. 4%. The main etiologies are discussed showing the part which is due to prenatal pathology and perinatal and postnatal pathology. Each chapter shows the role of biology and biochemistry in particular in the study of metabolic disorders of the nervous system. As far as genetic advice is concerned, it is not always easy as there still subsists a majority of cases of mental deficiency where the cause remains unknown. Research has an important role to play in the future if one wihes to cure or prevent mental deficiency.