In contemporary developmental science, relational development systems models have been used to frame the positive youth development (PYD) perspective, which posits that youth will thrive when there is alignment between their strengths and ecological resources in their context. Evidence from the 4-H Study of PYD indicates that out-of-school-time youth development programs are key ecological resources enhancing youth thriving. This chapter discusses the particular facets of youth development programs (the "Big Three"--positive and sustained adult-youth relationships, skill-building activities, and youth leadership opportunities) involved in promoting youth thriving. The importance of using theory to design and implement programs is also discussed, and challenges of reaching the diversity of American youth with effective programs are noted.