Introduction: Young people with intellectual disabilities (ID) are at an increased risk for experiencing mental health issues compared to their peers without disabilities. Further, there are limited resources available to help accurately assess mental health disorders and that are accessible for adolescents with ID. Method: This paper describes the iterative development and pilot testing of the Diagnostic Interview for Adolescents and Adults with Intellectual Disabilities (DIAAID). The authors utilized Evidence Center Design and Universal Design principals to develop the DIAAID; a multi-informant diagnostic interview. Results: The DIAAID development resulted in the creation of 15 adolescents disorder interviews and 24 caregiver disorder interviews. Preliminary results suggest that the DIAAID is a feasible and accessible diagnostic interview for adolescents with ID and their caregivers. Discussion: Lessons learned from DIAAID implementation and future areas research are discussed.