As siblings play an increasing role in the lives of people with intellectual and developmental disabilities as a result of the longer lifespan of this population and the aging of their parents, more investigative work in this area is warranted. To lay a foundation for this, the authors reviewed the English-language literature on siblings of adults with intellectual and developmental disabilities, focusing on three main questions: (1) what are the psychosocial outcomes of having a sibling with an intellectual and developmental disabilities on the sibling without a disability?; (2) what factors relate to the nature of the sibling relationship?; and (3) what factors relate to future planning, including expected and future relationships when parents can no longer provide care? This literature review looked at published work for the period 1970–2008 on adult siblings over 21 years of age that addressed relationships, psychosocial outcomes, and/or involvement in future planning. Twenty-three relevant studies were identified. Overall, these studies present a mixed, but generally positive picture of the psychosocial outcomes of having a sibling with a disability. They also indicate that siblings tend to have long-lasting close relationships with their siblings with a disability and anticipate taking on greater supportive roles as both grow older.