This innovative Research Handbook explores judicial, scholarly, and theoretical approaches to general principles in the EU legal order against the backdrop of considerable uncertainty about the concept. It does so by analysing both a diverse range of general principles in discrete areas of EU law ('zooming in') and external, wider perspectives on the notion of a general principle of law from international law, comparative law, and legal theory ('zooming out'). Rather than arguing for a single closed definition of what a general principle of law in the EU legal order must look like, this Research Handbook identifies conceptual, theoretical, and legal parameters within which the doctrine of general principles can be meaningfully discussed and contested in EU law. The different analytical layers built into this Research Handbook shed light on whether general principles are defined by the different contexts in which they apply; whether general principles are in practice leading to more coherence between different areas of EU law; and what challenges they create for the EU legal order. Chapters thus contribute to a more refined methodological and doctrinal understanding of general principles in the EU legal order. Opening up new spaces to critically reflect on the concept, role, significance, and limitations of general principles, the Research Handbook on General Principles in EU Law will be a key resource for scholars and students of European law, politics, and theory of integration and internationalisation.