This chapter presents a review of adaptive behavior assessment from conceptual, technical, and practical perspectives. Although adaptive behavior is a construct with relevance across multiple disability populations served by rehabilitation professionals, its greatest relevance concerns persons with intellectual disabilities (ID). This chapter presents adaptive behavior assessment within an ID context. It begins by describing the population of persons with ID and how they are defined through federal legislation and professional associations. Specific focus is placed on the growing importance of adaptive behavior in the process of identifying persons with this disability. The chapter then presents a review of standardized and informal approaches to adaptive behavior assessment. To illustrate its professional importance and use of best-practice approaches, the chapter then addresses three practice areas where adaptive behavior assessment plays a key role in contemporary practice with persons with ID, including death penalty evaluations, community-based habilitation, and culturally responsive assessment.