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Ching-chih Chen (1998) has argued that no cohesive or comprehensive theory of digital libraries has yet to be fully developed. Digital libraries include collections of books, journal articles, graphics, newspapers, and other material in digital format; in sum, collections of digitized content.
The Digital Library Federation offers one definition of digital libraries:
"Digital libraries are organizations that provide the resources, including the specialized staff, to select, structure, offer intellectual access to, interpret, distribute, preserve the integrity of, and ensure the persistence over time of collections of digital works so that they are readily and economically available for use by a defined community or set of communities." (Digital Library Foundation http://www.clir.org/diglib/dldefinition.htm).