Background: Doctors are increasingly expected to improve the health and well-being of populations, as well as to care for individuals. However, despite extensive efforts to integrate population health into undergraduate programmes, engaging students in such learning is notoriously challenging. Threshold concepts are transformative, integrative, irreversible and fundamental to understanding a discipline. Grasping such concepts requires learners to cross a liminal space, which often involves struggle. Methods: We employed a form of transactional curriculum enquiry, involving qualitative and quantitative methods, with experienced population health medical educators to identify and explore threshold and troublesome concepts in population health. Results: Attributing causality, inequalities in health and doctors’ responsibility for populations not just individuals were the concepts most participants thought were threshold. The value of qualitative research, health as politically and socially determined and not taking evidence at face value were the concepts ranked as most troublesome for learners. Participants found the notions of threshold and troublesome concepts helpful and empowering. They described ways these new ideas would influence how they taught population health. Discussion: Transactional curriculum enquiry can offer insights into which population health concepts may be threshold and troublesome. The number of such concepts identified in this study may help explain why students often struggle to engage in population health learning. Understanding which concepts are threshold and particularly which are troublesome can help teachers to better support learners and can also inform curriculum design. If our students are to … take responsibility for populations as well as individuals … it is vital that they cross these fundamental thresholds in learning.