In resource-limited countries, interventions to prevent mother-to-child HIV transmission (PMTCT) have not yet realized their full potential health impact, illustrating the common gap between the scientific proof of an intervention's efficacy and effectiveness and its successful implementation at scale into routine health services. For PMTCT, this gap results, in part, from inadequate adaptation of PMTCT interventions to the realities of the implementation environment, including client and health care worker behaviors and preferences, health care policies and systems, and infrastructure and resource constraints. Elimination of mother-to-child HIV transmission can only be achieved through understanding of key implementation barriers and successful adaptation of scientifically proven interventions to the local environment. Central to such efforts is implementation science (IS), which aims to investigate and address major bottlenecks that impede effective implementation and to test new approaches to identifying, understanding, and overcoming barriers to the adoption, adaptation, integration, scale-up, and sustainability of evidence-based interventions. Advancing IS will require deliberate and strategic efforts to facilitate collaboration, communication, and relationship-building among researchers, implementers, and policy-makers. To speed the translation of effective PMTCT interventions into practice and advance IS more broadly, the US National Institutes of Health, in collaboration with the President's Emergency Plan for AIDS Relief launched the National Institutes of Health/President's Emergency Plan for AIDS Relief PMTCT IS Alliance, comprised of IS researchers, PMTCT program implementers, and policy-makers as an innovative platform for interaction and coordination.