This collection of essays offers an overdue re-examination of some of the most important puzzles surrounding the idea of custom and its relationship to law. Although the title of the book suggests that the phenomenon for examination is 'customary law,' the essays within are most impressive as a series of insights into how custom itself should be understood, and what its implications are for legal theory. The book is best approached, and is most interesting, as an insight into the 'jurisprudence of custom,' albeit one which proceeds through discrete and sometimes only loosely connected essays. It examines what legal theorists, philosophers, and historians might want to know about custom; or what might puzzle them about custom, in contrast to the kinds of questions that anthropological, psychological, or sociological studies might ask.These questions include the thorny issue of the relationship between custom and law: is custom a source of law or a type of law? Does custom turn into law or law into custom? Can custom be authoritative and, if so, in what sense? They are the same questions that scholars in legal philosophy, legal history, and legal theory have yet to satisfactorily answer; sometimes because they have been given insufficient attention, but other times, because the attention they have received comes in approaches to law no longer considered convincing or desirable. Though most of these questions remain outstanding at the end of this book-which is not and does not claim to be a definitive account of custom or customary law-substantial progress is made in explaining the concept of custom, exploring the role of custom in the theories of key philosophers and common lawyers, and offering a way past the stagnant debates about the idea of customary international law. For anyone interested in attempting to build a jurisprudence of custom, this book serves as an indispensable provocation of some of the most pressing puzzles.