This book develops a systematic analysis of the principle of proportionality of penalties in EU law. Until now, the principle has been mainly assessed from a criminological point of view. By departing from existing literature, the book advances an original EU law perspective on proportionality of penalties, which underlies the analysis of its theorisation, place and influence on domestic criminal systems. Building upon this approach, scholarly contributions in the book systematically delve into the various implications of the principle in EU law, while paying particular attention to its profound interaction with criminal law concepts.