The Accelerating Children's HIV/AIDS Treatment (ACT) Initiative was an ambitious joint donor initiative to increase the number of HIV-positive children and adolescents on treatment over a 2-year period from 2014 to 2016. The funding was provided by the US Government's President's Emergency Plan for AIDS Relief (PEPFAR) and the private Children's Investment Fund Foundation (CIFF). Great gains were achieved across the 9 ACT focus countries in pediatric treatment coverage. This article assesses the status of sustainability in the ACT countries after the pediatric treatment surge using PEPFAR sustainability data and a CIFF independent evaluation of sustainability. Although a focus on treatment is critical for pediatric HIV and HIV broadly, there is also a need to support the host country ability to maintain the progress gained once donor funds and initiatives transition. It uses the case of the ACT Initiative to argue that although surge activities are successful in rapidly scaling treatment results, there are concerns related to the health system's ability to maintain the progress along the full cascade. It shares important lessons for planning for and management of transition to support future donor efforts in pediatric HIV, overall HIV programming, and broader global health initiatives.