ObjectiveOur purpose is to answer the following question: What mechanisms or components of programs, groups, or interventions improve psychological and social aspects of the lives of siblings of children who have intellectual/developmental disabilities (IDD)? Secondly, using a realist review format, we develop program theories and recommendations that can guide current and future sibling support programs.BackgroundThere is evidence that having a sibling with IDD can affect the physical health, behavior, and mental health of siblings who do not have IDD. As a result, support programs have been developed for these nondisabled siblings.MethodsWe conducted a realist review of 31 studies assessing support programs for siblings of children with IDD. Gray literature was also included. Applying the realist paradigm, we identified program contexts, mechanisms that promote program effectiveness, and the outcomes of the programs.ResultsWe found that contextual information was often lacking in the research and no studies examined the effectiveness of individual program components. However, the mechanisms of validation of feelings and experiences, time with parents, and respite were observed as beneficial for the siblings. From our analysis and using a realist review paradigm, we developed four program theories that reflect the siblings' experiences in the various programs.ConclusionThe four program theories derived from this realist review can be summarized as “I am not the only one,” “I have needs too,” “I need my parents' attention,” and “I need a break.”ImplicationsBased upon the four theories, we offer recommendations to guide program development.