Many health systems are in the midst of transformation. They are slowly moving from the delivery of reactive care focused on individuals to considering proactive ways of supporting the health and well-being of populations. However, the road to what is often called ‘population-health management’ is rife with implementation challenges. One type of implementation support that has been used to navigate these challenges is technical assistance. Though the use of technical assistance is well documented, there is no consensus on a clear definition or understanding of how it can be used to support system transformation. This thesis contributes to the field of technical assistance through three qualitative studies. First, a critical interpretive synthesis develops a definition and logic model for technical assistance. This logic model integrates diverse academic and grey literature. It aims to draw clearer boundaries around technical assistance as a concept and provide a common language for researchers, technical assistance providers, and decision-makers to use. Second, a qualitative descriptive study explores the use of technical assistance in population-health management transformations in England and the U.S., examining what technical assistance has been provided, by whom, and in what areas of application. Finally, a case study unpacks the use of technical assistance for a recent health-system transformation in Ontario. It examines the influence that political factors related to institutions, ideas, interests and external events have on shaping its evolution. Together, these three studies provide greater clarity on the use of technical assistance in health-system transformations and the range of factors that may affect how it is conceptualized and operationalized. / Thesis / Candidate in Philosophy