The past decades have witnessed tremendous changes in research and practice in the area of developmental disabilities. Advances in genetics, neuroimaging, and psychology have been tremendous. There have been marked changes in aspects of rehabilitation and treatment, and much more knowledge has been gained regarding mental disorders associated with mental retardation. As noted in the preface to this volume, even a decade ago the task seemed to be to convince mental health workers to pay attention to the psychiatric disorders of this population. Curiously, it was as if many clinicians and researchers believed that having mental retardation, if anything, seemed to "immunize" individuals against other problems. On the face of it, this made little sense, and epidemiological and other studies using rigorous methods have now shown that this group has a rate of mental disorders that is several times the rate in the general population. This being said, it is also clear that the disorders observed sometimes challenge our current nomenclature, particularly when they occur in the most severely handicapped individuals. On the other hand, it is also reasonably clear that our current diagnostic systems work reasonably well among those with mild mental retardation.