Intellectual disability (ID) is defined by impairments in intellectual and adaptive functioning. As such, tools designed to assess these domains would theoretically be ideal outcome measures for treatment trials targeting core symptoms of ID. However, measures of intellectual and adaptive functioning have rarely been used as primary outcome measures to date and further study is needed regarding their usefulness to measure change. This area of inquiry is important because promising, mechanism-modifying treatments for conditions leading to ID are being initiated. To show efficacy, these treatments need to demonstrate an impact on core features of ID. After reviewing literature on this topic, we suggest solutions to several problems outlined, including use of out-of-age-range testing, alternative metrics, and development of new measures.