This book is about the principle of legality in European criminal law. It is divided into three Parts and eight Chapters. The first Part is introductory and it is comprised of two Chapters, where the research topic and the methodology are set out (Chapter I and II respectively).

Chapter 1 introduces and explains the central topic of this research. The principle of legality is a keystone doctrine of national criminal justice systems. In short, the principle means that there is no crime without law. It has two functions; it legitimises the use of criminal law powers by the state (legitimising function) and it influences the operation of criminal law normatively (normative function). This principle legitimises the use of state powers by defining the concept of law as opposed to state force. It concretises the application of other important values or principles, which are often considered the theoretical rationales of the principle of legality. These include the Rule of Law, the separation of powers, democracy, and the principle of individual liberty and autonomy. Chapter 1 highlights that this principle may elicit different interpretations, depending on how one defines its legitimising role. Normatively, the principle of legality defines the sources of criminal liability and it guides the use of the discretionary powers by state actors. Four prohibitions stem from it, namely the prohibition of customary criminal liability, of retroactive criminalisation, of vaguely prescribed behaviour and finally, the prohibition of extensive interpretation by courts.

Whereas the legality principle is fundamentally important for the criminal justice systems, its interpretation and application in European criminal law is incoherent and incomplete. European criminal law is an area still in progress and increasingly more criminal law powers are entrusted to European state actors. Chapter I explains further the realm of European criminal law. The existence and nature of these powers demand that they are employed legitimately. The current application of the principle of legality in EU law is fragmentary. Many of its normative functions have already been developed judicially but these do not apply coherently at all instances. More importantly though, there has been very little discussion on the legitimation of the European criminal justice system and the conditions under which European criminal law is distinguished from state violence.