Given the new and challenging academic and social structures with which children entering adolescence and young adulthood must contend, adaptive skills become increasingly important during this period of development. This chapter discusses adaptive behavior as it is related to adolescents and young adults, the validity of the Adaptive Behavior Assessment System-II (ABAS-II) with adolescents and young adults, and a psychoeducational case study that includes ABAS-II data from multiple respondents. As children enter adolescence, the everyday tasks of life become different and more complex and adaptive skills must grow and change to address the tasks. The challenge of the secondary education environment becomes a particular concern as demands are increased. Similarly, obtaining and maintaining employment becomes more of a concern during the later stages of adolescent and young adult development than in earlier stages. Adolescents and young adults, both with and without various disabilities and disorders, can exhibit adaptive behavior deficits as well as maladaptive behavior problems. There are a number of disorders and disabilities that affect adaptive behavior as well as cause maladaptive behavior problems for individuals, and have a relatively high onset in and particular salience to adolescence and young adulthood. To this end, it is particularly important that adaptive behavior be assessed regularly when working with adolescents and young adults with disabilities, disorders, and other problems that contribute to deficits in adaptive functioning. © 2008 Elsevier Inc. All rights reserved.