In the United States alone, there are almost 300,000 individuals with developmental disabilities. Researchers have focused primarily on such individuals and their parents; however, the developmental outcomes of such individuals' siblings have been understudied. Research on siblings frequently is atheoretical, which makes it difficult to organize findings in ways that enhance understanding of the multiple influences on sibling development and to predict and promote siblings' outcomes. Application of a theoretical framework is crucial, as it has implications for supports available to siblings and families, as well as for research, programs, and policy. Here, a bioecological framework is used to organize existing research about siblings of individuals with disabilities and the factors that help or hinder their development and to highlight areas needing further study.