Millions of children and adolescents participate in organized sports and physical activities worldwide. Many in the general public, including parents, coaches, and policymakers, believe that participating in sport enables opportunities for youth to gain confidence, learn teamwork, and attain life lessons. However, positive outcomes are not an automatic consequence of participating in sport, illuminated by reports of negative coaching behaviors, overinvolved parents, and competitive rivalries with teammates and opponents. The potential for sport to improve youth participants’ perceptions of competence, relatedness, enjoyment, and self-determined motivation is highly dependent on the quality of interactions and relationships with important adults and peers. This chapter reviews the literature on motivational processes in youth sport and physical activity with a specific focus on beliefs and behaviors by parents, siblings, peers, and coaches that promote and sustain positive psychosocial and behavioral outcomes for youth participants. Robust findings are translated to evidence-based best practices for stakeholders in youth sport, and future research directions are offered to delve further into the conditions that undergird positive motivational outcomes in youth sport and physical activity.