Resources to promote positive development in young people have been identified in many contexts such as families, schools, neighborhoods, and out-of-school time programs. The most important resources within these contexts are the relationships that young people have with committed, caring adults. In this chapter, we provide an overview of research, based on findings from the 4-H Study of Positive Youth Development (PYD) and other scholarly work, on the essential role of adults in promoting PYD. We provide examples of organizations that have applied this research to their programming and practices to impact the well-being of young people. Finally, we provide priorities for practice and policy based on these findings. Specifically, we recommend that programs promote positive parenting behaviors, build cultural competence among their staff, engage parents more fully in programs, train staff and the youth in relationship-building skills, and thoughtfully consider how they create mentoring matches. Policy makers should invest in creating supportive youth systems in which youth voices are heard, such as family support programs and comprehensive community initiatives, to promote youth thriving.