This dissertation examines how youth expertise is conceptualized, valued, and actualized within a high school afterschool program run through a center for university-community partnership. Youth expertise exists at the intersection of youth voice/participation literature and theories of expertise. Youth voice initiatives often function as tokenistic efforts that fail to genuinely engage young people's knowledge, while youth advocacy frameworks like youth-led participatory action research and youth organizing provide stronger models for centering youth perspectives. While literature on youth advocacy espouses a strong underlying value of youth expertise, the concept remains undertheorized. Traditional Western conceptions of expertise focus on individual credentials and specialized knowledge. More recent scholarship on expertise has emphasized social processes, collaborative knowledge production, and knowledge held by those directly affected by problems but without formal credentials. Drawing from these lines of scholarship and that of Black feminist and indigenous epistemologists, a theory of youth expertise challenges adultist assumptions by recognizing young people as legitimate sources of knowledge. The eight-month study employed participant observation, semi-structured interviews with adult staff and volunteers, focus groups with high school participants, and document review of program materials. The research identifies an important tension between organizational values and implementation practices. While the university center leadership espouses a value of youth expertise, the organization lacks formalized systems specifically designed to support it. Consequently, the successful recognition and utilization of youth expertise depends largely on the dedication of individual staff members who create spaces where young people's knowledge is valued in practice. Youth expertise manifests in three interconnected dimensions. First, it is relational, emerging through trusting relationships in which adults intentionally disrupt normative age-based hierarchies. Second, youth expertise is experiential, grounded in young people's direct lived experiences. Third, youth expertise is perspectival, offering vantage points that are not accessible by adults. This framework positions youth expertise not as a developmental steppingstone toward adult expertise, but as a legitimate epistemological resource that enhances collaborative knowledge production. The findings suggest implications for educational practice, organizational structures supporting youth-adult collaboration, and broader conceptions of expertise.