What is the place of principles in law? This is the fundamental question that this book seeks to answer. Based on a historical-genealogical study, it addresses the main theoretical currents of Western legal thought while seeking to clarify the meaning of what has become known as ""principles of law"". In the wake of this investigation, the book also analyses the two main legal systems of the West and the way in which tradition has treated principled formulations in the history of law. Starting with ancient natural law and ending with movements that attempted to overcome legal positivism, this book is of interest to anyone who wishes to better understand the historical reasons for the increasingly frequent inclusion of abstract and evaluative norms in the laws of contemporary societies. The book is intended for law students and professionals in the areas of Legal Theory and Philosophy, Legal History, and Constitutional Law.