Developing a European Health Union necessarily involves re-examining European Union (EU) and national level interaction. While this is currently framed around limited legal competence, its political identity is notable: the EU’s role is restricted because health is a Member State competence. Although this has not precluded more subtle EU-level involvement via, for example, internal market rules and fiscal policies such as the European Semester, the explicit development of a European Health Union is more contentious and political by nature. We consider here how the principle of subsidiarity and concerns about national sovereignty may help – or hinder – the shaping of a true European Health Union.