Despite being recognised as complementary to formal learning, informal learning is not yet well understood. Using a qualitative synthesis approach, we systematically reviewed how students' informal learning interactions are represented in health professions education research. Thirty-five articles were identified. Our three-stage analysis identified five themes: (1) conceptualisations and theoretical underpinning; (2) the what of informal learning; (3) contextualised and relational (the where and how); (4) enablers and threats; and (5) capabilities and dispositions that help make learning happen. The findings suggest that scholars need deeper engagement with concepts and theories related to informal learning; and greater attention is required to develop learner capabilities and dispositions, such as agency. Furthermore, there needs to be an increase in the connections between formal and informal learning, as the content often left to chance opportunities in informal learning contexts is too important not to be designed for and embedded in formal education.