'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