The bibliographic work has assumed a great deal of importance in modern cataloging. But the concept of the work has existed for over a century, and even some of the earliest catalog codes differentiate between the intellectual work and its instances. This article will delve into the history and theory of the work, providing a basic overview of the concept as well as a summary of the myriad uses of the work throughout the history of cataloging. In addition to monographs, this paper will look at the work as applied to music, moving images, serials, and aggregates.