This book discusses legality, one of the four main principles of criminal law, and is divided into six parts, according to the scientific understanding of this key concept. Chapter 1 explores the relation between legality and the general theory of criminal law in the context of the structure and development of legality in human society. This chapter also outlines the four secondary principles of legality and describes them in general terms. Chapters 2-5 discuss in detail each of the four secondary principles (Legitimate Sources of the Criminal Norm; Applicability of the Criminal Norm in Time; Applicability of the Criminal Norm in Place; and Interpretation of the Criminal Norm). Finally, Chapter 6 rounds out the discussion by addressing the problem of the conflict of laws.