L. S. Vygotsky laid the foundation for the uniting of different directions in the studies of developmental anomalies into special psychology as a branch of psychological science. These foundations are: special features of mental activity characteristic for different types of developmental anomalies; marking out of study objects and working out of special methods of examination. L. S. Vygotsky's ideas make possible the realization of a new approach to differentiated psychological diagnostics of developmental anomalies.