Insights from behavioural science can inform a wide range of solutions including policy reforms, infrastructure changes, process reengineering, communications, devices and others. Its application in global health has, however, often focused on narrowly defined issues with linear theories of change that are relatively straightforward to study. Using behavioural science to examine 'upstream' actors in complex health systems holds promise as a complementary and underused approach to improving health systems. Behavioural scientists have missed opportunities to tackle systemic issues and the field of health systems strengthening rarely accounts for human behaviour at the macrolevel of health systems. We present a framework, developed by experts from a range of disciplinary backgrounds in global health and behavioural science that guides (1) the distillation of a health system challenge into concrete, addressable behaviours (specific actors and accompanying actions) and (2) the investigation of contextual factors that influence each behaviour.