Training of severely mentally retarded institutionalized patients in basic social skills is one of the most important and difficult problems confronting those concerned with habilitating these individuals. This chapter describes the application of operant conditioning techniques in toilet training of severely retarded boys at the Columbus State School. It aims to test the efficacy of operant conditioning techniques in toilet training of severely mentally retarded boys. Toilet training proper was started after two weeks, when initial confusion subsided and it appeared that the Ss had adapted to the training unit environment. Each of the three groups: the operant conditioning group, the conventional training group, and the control group, were observed before and after training. Observations were made independently by raters who, for 512 hours each day for three consecutive days, recorded the frequency of all incidents of incontinence and use of bathroom facilities for each child.