The transition to adulthood for young adults with intellectual and developmental disabilities (IDD) is known for its unique challenges, such as limited or non-existent services after high school and poor outcomes faced in adulthood. Given these challenges, it is important to further explore the transition planning experiences of caregivers and young adults with and without IDD, Individualized Transition Program (ITP) involvement and satisfaction, and predictors of adulthood outcomes. This study used qualitative data from caregiver (n=116) and young adult (n=99) interviews to further explore the needed support and existing support during the transition from high school into adulthood. Thematic analysis results found that across diagnostic groups (IDD vs. neurotypical), reports were similar regarding support needed and existing support provided. Supports needed included employment/career planning, adaptive skills, transition involvement, and financial education. One support needed that was unique to the IDD group was the accessibility of services/information. Existing supports included social support, adaptive skills, employment/career preparation, and transition-specific preparation. These existing supports overlapped with needed supports, indicating a need to enhance and coordinate these supports across different domains, not just within the home but at school and through other government-funded agencies. ITP results found that over 96% of caregivers reported involvement in the ITP process, but only 59% reported being satisfied with the ITP. Over 66% of young adults were reportedly involved in their own ITP, and about 59% actively participated. As for adolescent predictors of transition outcomes in early adulthood, externalizing behaviors at age 15, social skills at age 15, IQ at age 13, and diagnostic category were all significant predictors of adulthood outcomes. These findings may provide insight into how to individualize transition support for young adults with IDD. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]