Approximately one quarter of American adults have at least one disability that influences their ability to participate in activities of daily life. Individuals with disabilities beginning in childhood may experience reduced social opportunities as they age. Social capital, the resources exchanged among individuals through their relationships with each other, is a key component to understanding how people experience the world around them. This dissertation describes the findings from three studies examining the research question: "how do people with developmental disabilities experience social capital from childhood to young adulthood?" The first study follows Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standards for a scoping literature review to investigate the ways social capital is currently conceptualized and applied to developmental disability research from childhood to emerging adulthood (ages 5-29 years). Results describe how social capital is defined, measured and applied to disability research and identify gaps in the literature. This study addresses the gap in our knowledge caused by the dispersion of social capital and developmental disability literatures across many disciplines. This review also identifies measurements and proxies for social capital in extant literature, informing the conceptualization and application of social capital in my other two dissertation studies. Results help us understand the current knowledge of social capital for people with developmental disabilities and areas in need of further research. The second study describes results of an analysis of extracurricular participation rates between children with and without DD and the associations of childhood-adolescent extracurricular participation with mental health outcomes in young adulthood. Results show differences in the rates of extracurricular activity participation between children with and without developmental disabilities. Results also show that extracurricular activity is associated with lower psychological distress and higher levels of flourishing among individuals with and without developmental disabilities. The third study addresses, "for young adults with intellectual and developmental disabilities, what does it mean to be interdependent?" Using photovoice methods, college students with intellectual and developmental disabilities described their experience of interdependence with their photos and stories. Participants described their openness to being helped, the role of their families in forming and maintaining interdependence, the role of others as they do new and challenging things, and the perceived vulnerability associated with helping others. Taken as a whole, these studies extend our understanding of the role of social capital in the lives of people with disabilities by: mapping the current definition, measurement, and application of social capital in the developmental disability literature; using a nationally representative, longitudinal dataset including people with and without developmental disabilities to highlight a population that is often hidden in nationally representative data; and including the voices of people with intellectual and developmental disabilities describing their lived experience of interdependence. Recognizing the role of social capital in the lives of individuals with developmental disabilities helps to identify needed policies and supports that may enhance the overall quality of life for individuals with developmental disabilities throughout their lives. This research serves as a stepping stone toward a more inclusive and supportive society for individuals with developmental disabilities, highlighting their agency and contributions in developing social capital and fostering interdependent relationships.