This article examines the principle of legality, a principle of statutory interpretation that requires clear statutory words to oust basic common-law norms. The principle is of growing importance in the Supreme Court's public law jurisprudence, yet it has garnered little scholarly attention. This article offers a comprehensive account of the principle, unpacking its core elements and identifying key controversies. The article reveals that lying beyond this apparently straightforward principle is a complex and elaborate jurisprudence, which raises fundamental issues of principle, policy and judicial legitimacy.