Instant electronic access to digital information is the single most distinguishing attribute of the information age. The elaborate retrieval mechanisms that support such access are a product of technology. But technology is not enough. The effectiveness of a system for accessing information is a direct function of the intelligence put into organizing it. Just as the practical field of engineering has theoretical physics as its underlying base, the design of systems for organizing information rests on an intellectual foundation. The subject of this book is the systematized body of knowledge that constitutes this foundation.