The digital library universe is a complex framework. The growth and evolution of this framework in terms of approaches, solutions and systems, has led to the need of common foundations capable to set the ground for better understanding, communicating, and stimulating further evolutions in this area. The DELOS Digital Library Reference Model aims at contributing to the creation of such foundations. It exploits the collective understanding that has been acquired on Digital Libraries by European research groups active in the Digital Library field throughout many years, both within the DELOS Network of Excellence and outside, as well as by other groups around the world. It identifies the set of concepts and relationships among them characterising the essence of the digital library universe. This model has to be considered as the map allowing various Digital Library involved players to follow the same route and share a common understanding in dealing with the entities of such universe. This volume presents the DELOS Reference Model by introducing the principles governing it as well as the set of concepts and relationships that collectively capture the intrinsic nature of the various entities of the digital library universe. Because of the broad coverage of the digital library universe, its evolving nature, and the lack of any previous agreement on its foundations, the Reference Model necessarily is a living framework, so is this document to be considered. Continuous evolutions are envisaged in order to gain a number of well formed and consolidated definitions, shared by the digital library community.