This article updates the current conceptualization, measurement, and use of the adaptive behavior construct. Major sections of the article address an understanding of the construct, the current approaches to its measurement, four assessment issues and challenges related to the use of adaptive behavior information for the diagnosis of intellectual disability, and two future issues regarding the relations of adaptive behavior to multidimensional models of personal competence and the distribution of adaptive behavior scores. An understanding of the construct of adaptive behavior and its measurement is critical to clinicians and practitioners in the field because of its role in understanding the phenomenon of intellectual disability, diagnosing a person with intellectual disability, providing a framework for person-referenced education and habilitation goals, and focusing on an essential dimension of human functioning.