Towards an agenda for implementation science in global health: there is nothing more practical than good (social science) theories

### Summary box Ridde recently called for a commitment to implementation science and for ‘better quality research in order to have a greater understanding of how to implement health interventions’.1 He argued for better use of theory and implementation science studies that contribute to theory. This is indeed of paramount importance, especially for interventions in complex settings such as health systems. In practice, however, few researchers take up the challenge; all too often, implementation studies adopt a mixed methods approach that is a-theoretical. If there is a lack of theory in the field of implementation science, it is not for want of theories. In the case of health governance, for instance, theory development and empirical research testing such theories have been taking place in the fields of economics and political science. However, there has been little, if any, systematic crossing over of recent governance theories to the field of health policy and systems research. This does not only apply to health (system) governance; research on health worker motivation and strategic decision-making at operational level, for instance, could use a healthy injection of recent theories and methods from other disciplines. In applied research fields, such as global environmental studies, there are more signs of effective cross-pollination. The study of global climate change is an example of how the integration of different paradigms yields new theoretical insights that drive empirical research; for example, the late Elinor Ostrom's conceptualisation of polycentricity in efforts to address global …