While research on program effectiveness offers ample evidence that afterschool programs can benefit young people in a variety of ways, this same body of research demonstrates that not all programs are equally effective (Granger, 2008). Some programs show positive results in many or all major outcome categories. Other programs are associated with positive results for some but not all outcomes; still others show no positive outcomes at all. This variability in effectiveness highlights the need for a better understanding of "how" successful afterschool programs actually work, while keeping in mind the ever-present question of how to define success. This review article examines formulations of afterschool program quality emerging from outcome research, expert and professional consensus, and "process" research on how successful afterschool programs operate. After briefly summarizing current literature on the effectiveness of afterschool programs, the authors review empirical evidence regarding the characteristics of effective programs, that is, what effective afterschool programs do to produce positive outcomes. Next, they provide an overview of how program quality has been conceptualized by afterschool researchers. We conclude by discussing the field's emerging consensus regarding quality afterschool programming, highlighting key considerations as the afterschool field undertakes efforts to achieve both program quality and positive outcomes for youth.