Background: Declines in physical activity (PA) in children and youth have contributed to increases in childhood overweight and obesity. The Safe Routes to School (SRTS) program was developed to promote school active transportation (AT) and reverse the trend. Methods: Adopting concepts of a realist approach, this article seeks to understand strategies of adoption that worked in the Canadian and United States context. Inclusion criteria consisted of adoption of SRTS program, identification and definition of SRTS, implementation in Canada /United States, and partnership identified. Results: Partnerships focused on increasing the number of children using AT to school. With unique political and funding atmospheres, a common strategy was developing multilevel comprehensive partnerships to mobilize knowledge and resources, as well as to align intervention planning. Key successes, tools used to measure success, as well as benefits, challenges and lessons learned from partnerships were identified. Conclusion:This article is the first attempt to examine SRTS at the state/provincial/city level to understand key adoption strategies using a realist approach. It found collaborative community-research partnerships that initiated SRTS and created cultural shifts in communities from the individual to policy level. Researchers, schools and communities interested in increasing school AT should consider SRTS as a valuable approach.