Revision with unchanged content. Today, digital libraries comprise a wide range of systems and services from classic libraries and electronic encyclopaedias to computer-supported collaborative work environments, image databases, and documentation systems. Recently, novel forms of digital libraries were developed and introduced to a wide audience on the World Wide Web: wikis, weblogs, social networks, file sharing services, etc. These types of libraries have unconventional and sometimes chaotic approaches to authoring and organising content. Although they manage to attract millions of users, many of their characteristics are not well understood, and some of their implications might not even have been discovered yet. In this book, three particular aspects of digital libraries are investigated. First, electronic encyclopaedias as a popular application area of libraries are addressed. Then, the use of metadata is discussed and finally, collaborative features in libraries and functionality known from knowledge management systems are highlighted.