This Manual contains the most current and authoritative information and knowledge on intellectual disability, including best practice guidelines on diagnosing and classifying intellectual disability and developing a system of supports for people living with an intellectual disability. Written by a committee of 18 experts, "Intellectual Disability: Definition, Classification, and Systems of Supports" (11th edition) is based on seven years of work on: (1) a synthesis of current information and best practices regarding intellectual disability; (2) numerous reviews and critiques of the 10th edition of the American Association on Intellectual and Developmental Disabilities (AAIDD) definition manual; and (3) feedback from the field regarding a series of articles published by the Committee. This is the first official AAIDD definition manual with the terminology "Intellectual Disability" (formerly mental retardation).