This book explores several key ideas relating to the 'principle of legality'.

In the United Kingdom, the 'principle of legality' is an interpretive tool applied by courts to legislation that is deemed to be unclear. The term is used as shorthand for the idea that Parliament may only override fundamental legal or constitutional norms using clear legislation. This deceptively simple notion has a rich and complex historical pedigree. Since the end of the twentieth century, a 'breach of the principle of legality' has become a discrete ground of judicial review rooted in the common law. But its status, definition, and reach all remain deeply contested. In recent years, recourse to 'the principle of legality' in public law claims has become increasingly controversial. As the corpus of case law has grown, the application of the 'principle of legality' has been identified by judges, academics, and politicians as both a source of legal uncertainty in its own right, and a vehicle for potential judicial overreach.

By building on a rigorous analysis of relevant case law, this book considers the intellectual foundations of 'the principle of legality'. As the analysis unfolds, we will see that there is no singular 'principle of legality' which we can accurately pinpoint. Instead, it makes sense to talk about multiple principles of legality, doing distinct, but related, work. This becomes apparent when we explore the history of the principles of legality in the context of not just public law, but the wider legal system.

Moving beyond the historical context, the book explores where and how the principles of legality apply in the contemporary legal system. Subsequent chapters explore the difficulties and uncertainties of determining whether the principles of legality are engaged. Thereafter, attention turns to what norms legality does and does not protect and how courts decide upon this. The final chapter sets the principles of legality in their broader constitutional context. Overall, it will demonstrate that concerns about relating to judicial overreach are overstated. In fact, the reverse is true: the application of the principles of legality do introduce a dimension of legal uncertainty, but ultimately fall short of their constitutional potential.