Instead of the usual apologetic treatment found in legal doctrine, linked to the determinacy, immutability or predictability of norms, this book treats legal certainty innovatively, holistically and in depth. Using a method at once analytical and functional, Professor Avila examines the structural elements of legal certainty, from its definition and foundations to its various dimensions, normative forces and efficacies, citing a wealth of examples from case law to support each of the theses defended. No subject is more important and topical than legal certainty. Problems relating to lack of understanding, instability and unpredictability of law intensify day by day everywhere, in civil law and common law countries alike. Normative sources are increasingly diverse in origin (national, international, community) and multiple in nature (legal, contractual, jurisprudential). They change constantly, and present increasingly frequent problems of ambiguity and vagueness that significantly hinder their comprehension. This state of affairs, which to a greater or lesser extent is true of any legal order, justifies a return to the subject of legal certainty. In this book, essential questions are answered such as: Legal certainty in what sense? Certainty of what, for whom, in whose vision and by whom? When, to what extent, and to what end? "(...) it is probably the most comprehensive and systematic study ever produced on this subject using the analytical method." (Riccardo Guastini, Professor of Jurisprudence, University of Genoa, Italy)