This dissertation is composed of four stand-alone yet linked chapters. Chapter 1 provides an overview of the theoretical underpinnings of the constructs of adaptive behavior and support needs and their evolution throughout history. There is also a review of the limited literature available examining the relationship between the two constructs that demonstrates the incongruity among outcomes. Chapter 2 examines the construct validity of two new instruments developed by the American Association on Intellectual and Developmental Disabilities the "Diagnostic Adaptive Behavior Scale" ("DABS") and the "Supports Intensity Scale for Children (Field Test Version 1.1)" ("SIS-Children"). The chapter provides evidence for the construct validity of the two instruments to measure the prospective constructs of adaptive behavior and support needs through confirmatory factor analysis. Chapter 3 builds on the results from Chapter 2 and goes one step further by examining the theoretical relationship between the constructs of adaptive behavior and support needs in children ages 5-16 with intellectual disability through structural equation modeling. The findings suggest that while the two constructs are intimately related, they do in-fact represent two distinct constructs. Finally, Chapter 4 summarizes the previous chapters and draws from them conclusions and implications for future research, policy, and practice for children with intellectual and developmental disabilities. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]