The chapter explores the principle of the rule of law within the European Union’s supranational multi-level system, examining its development and application across national and supranational levels. It argues that the rule of law ensures state actors’ adherence to legal frameworks, preventing arbitrary governance and fostering trust among EU member states. The analysis highlights variations in national interpretations, such as Germany’s constitution-centric approach and the UK’s reliance on common law. At the supranational level, the EU’s rule of law builds on treaties, the Court of Justice of the European Union’s jurisprudence, and shared legal heritage, encompassing principles like proportionality, legal certainty, and fundamental rights. By addressing challenges to the rule of law in some member states, the chapter sketches the EU’s response and hypothesizes it as vital for maintaining cohesion and legal integrity in its complex multi-level governance.