Peer-Tutoring is a well established cost effective method of teaching, but most of such studies used non-disabled children as tutor, tutees may be either non-disabled or disabled children. Present book is based on an experimental study in which children with mental retardation taught number skills to their retarded juniors. Students with mental retardation attending pre-vocational class served as tutors and tutees were from primary class. Tutors were selected on the basis of their abilities to handle younger children, their social skills and their ability to communicate as observed through several checklists prepared for this purpose. Initially tutors received training for twenty sessions and then they taught tutees individually for twenty sessions. The result revealed a significant gain in number skill of tutees, and improved social and communication skills of peer-tutors. It was also evident from the findings that competencies could be improved in children with Mental Retardation to be effective tutor for younger children with mental retardation.