This report provides an overview of knowledge organization systems (KOSs) and includes pertinent examples of their application to digital materials, offering extensive practical information for institutions embarking on digital library initiatives. The first section of the report defines the general characteristics of KOSs, with emphasis on their connection to a particular view of the world. The historic origins and uses of KOSs, in libraries and in other information management environments, are described, and various types of KOSs are discussed. The second section provides examples of how KOSs can be used to enhance digital libraries in a variety of disciplines, and describes how a KOS can be used to link a digital resource to related material. The third section discusses how KOSs can be used to provide disparate communities with access to digital library resources by using a KOS to provide alternate subject access, to add a new mode of access to the digital library, to provide multilingual access, or to support free-text searching. The report concludes with a discussion of issues to consider when using KOSs with digital libraries. It provides a framework for the design, planning, implementation, and maintenance of KOSs in digital libraries. (Contains 36 references.) (AEF)