This chapter focuses on the sustainment of evidence-based interventions for children and adolescents following the scale-up process. We first discuss some of the traditional assumptions governing implementation and scale-up and why they may not effectively support the sustainment of interventions, given limitations on the existing evidence base and expectations of the need for contexts to change to accommodate the intervention. We then argue for a new set of assumptions that center on the necessary fit between interventions and context to optimize sustainment and evolution of the interventions to maximize benefit. With these new assumptions in mind, we describe the implications toward reconsidering sustainability and scale-up of interventions and discuss opportunities for learning, data requirements to continuously improve interventions over time, and a process for sustaining impact over time.