When the growth of the movement to establish special classes for children who are subnormal in intelligence there has grown the demand for teachers who are trained in the psychology and education of such children. It is my hope that this volume, which consists of lectures given for some years past in Teachers College, Columbia University, may in a measure serve the purpose of such teachers. It is addressed primarily to them, rather than to clinicians, for whom the majority of books on this subject have been hitherto especially prepared. The discussion is, therefore, confined largely to the educational psychology of mentally deficient children, with relatively slight emphasis on methods of identification and diagnosis. The illustrative matter, also, relates to the achievement of defectives, rather than to their clinical features.<br><br>There has been a very conscientious effort to base the discussion soundly upon the results of psychological research, as set forth in the references appended. The lists of references are selected, not complete, except in the case of chapters on the psychology of development and learning. On these subjects the purpose has been to refer to all published experimental research bearing directly upon defectives.