In this study, we examined the characteristics of adolescents' deep interests or "sparks," the role of relationships in supporting the development of sparks, and whether having a spark was associated with positive developmental outcomes. Participants included 1,860 15 years olds from across the United States who participated in the national Teen Voice survey (56% European American, N = 1,860). Profile-centered analyses suggested that sparks are characterized by the intensity of positive feelings, immersion, and utility. The strongest sparks were associated with pursuits requiring more interpersonal engagement, such as sports, drama and dance, participating in politics, and serving others. Spark intensity was related to better social, academic, and affective outcomes. Additionally, youth with stronger sparks reported more encouragement, financial support, and transportation to spark activities from parents, mentors, extended-family, neighbors, school-based adults, and peers. Benefits of adolescents' engagement in interest-driven activities and the role of caring relationships in supporting such interests are highlighted.