The concept of mental retardation varies in time and place. Few children received education at senior school age until the end of the Nineteenth Century, so educational demands were not exacting. Today, these demands are more stringent and children who cannot meet them are, ipso facto, mentally retarded. Age determines whether society applies the criterion of educability or em-ployability, and the quality and stringency of these demands vary from community to community and from one historical era to another. Mental retardation must, therefore, be defined in terms, both of the capacity of the subject and the demands of society. The social environment may also influenae mental development directly. The intellectual and emotional climate of the home and the size of the family affect it. There are also combined hereditary and environmental effects that may influence the distribution of intelligence such as, for instance, the tendency of the more intelligent to migrate to areas or countries of greater promise. Our information about intelligence is restricted by the limitations of the instruments of measurement. The Intelligence Quotient is valuable in so far as it enables us to assess a subject's capacity to meet the demands of society during his lifetime, and in so far as it enables us to predict a child's future mental development. A limitation of tests for the very young is that the abilities tested are qualitatively different from those tested at a later age. We are dependent upon the assumption of a positive correlation between progress in such physical achievements as sitting up unaided and walking, and the more strictly intellectual functions tested at a later age. Mental retardation involves many disciplines. In this paper, we have confined ourselves to some sociological and psychological observations.