Education provides people with skills and resources that are important for many aspects of life, including health. Yet, despite substantial educational expansion, health inequalities remain a persistent challenge. The project FIELDS starts from the idea that in mass education systems, where more people attain higher levels of education, it is not only important how much education people have, but also what kind they choose. For instance, whether someone studies law, nursing, or engineering can shape life in very different ways. Surprisingly, study majors and vocational programmes have rarely been considered as factors influencing health. FIELDS investigates how and through which mechanisms fields of education shape health across the life course, moving beyond the well-known effects of educational level. Drawing on human capital and socialisation theory, it assumes that fields of education affect health through three main drivers: (1) opportunities to find gainful and healthy work, (2) tendencies to adopt health-promoting or harmful behaviours, and (3) the quantity and quality of social interactions. To examine these processes, FIELDS uses six of the world’s longest-running household panel surveys from Germany, the United States, the Netherlands, Switzerland, South Korea, and Australia. It will harmonise these data to conduct the first comparative life-course analyses linking field choices in youth to physical and mental health later in life and to examine how these links are mediated by work, behaviour, and social capital. Because individuals do not enter fields randomly but based on personal characteristics and structural constraints that also shape health, FIELDS applies a rigorous design using panel data, within-person estimation, statistical matching, and counterfactual mediation analysis to minimise bias. The project will open a new perspective on educational inequalities in health and help identify vulnerable groups for more effective interventions.