The theory of natural dualities, as presented in this text, is broad enough to encompass many known dualities through a rich assortment of substantive theorems yet concrete enough to be used to generate an array of previously undiscovered dualities. This text will serve as a user manual for algebraists, for category theorists and for those who use algebra in their work, particularly mathematicians and computer scientists interested in non-classical logics. It will also give the specialist a complete account of the foundations, leading to the research frontier of this rapidly developing field. As the first text devoted to the theory of Natural Dualities, it provides an efficient path through a large body of results, examples and applications in this subject which is otherwise available only in scattered research papers. To enable the book to be used in courses, each chapter ends with an extensive exercise set. Several fundamental unsolved problems are included. "One of the aims of the theory of natural dualities is to provide a systematic framework for making algebraic abstractions concrete. This book, written for those working in algebra, whether in the classical vein or with an eye towards applications to other parts of mathematics and to computer science, is the first thorough exposition of duality theory. Here you will find a clear, engaging, and accessible account of all its fundamental methods and key examples, as well as detailed and well-motivated discussions of intriguing and significant open problems. This well written book will be a valuable resource for algebraists, and an invaluable resource for those determined to advance the current flowering of duality theory itself." George McNulty, University of South Carolina First text in subject; aimed at algebraists, category theorists in mathematics and computer science.