This academic work offers a comprehensive analysis of the principle of legality as a foundational concept in criminal law. In the first chapter, the author examines the historical development of the legality principle, tracing its evolution from classical legal doctrines to its modern embodiment in national and international legal systems. The chapter further elaborates on the structural components and legal significance of the principle, including the requirements of clarity, non-retroactivity, and strict construction of criminal laws. The second chapter focuses on the operation of criminal law over time and in space. It discusses key issues such as the application of the criminal law to past and future acts, exceptions to the non-retroactivity rule, and the territorial, extraterritorial, and personal principles governing jurisdiction. The third chapter analyzes the sources of criminal law, emphasizing the role of statutory law, legal norms, and the necessity for consistent interpretation. The discussion concludes with an exploration of conflicts between criminal norms and methods of their resolution. This monograph is intended for researchers, academics, legal practitioners, and anyone interested in the fundamental issues of criminal law, its theoretical foundations, and practical applications. Recommended for publication by the statement of the Council of Research ethics of Tashkent State Law University No. 8 of June 20, 2025.