Digital libraries can vary immensely in size and scope, and can be maintained by individuals or organizations. The digital content may be stored locally, or accessed remotely via computer networks. These information retrieval systems are able to exchange information with each other through interoperability and sustainability.
The early history of digital libraries is poorly documented, but several key thinkers are connected to the emergence of this concept. Predecessors include Paul Otlet and Henri La Fontaine's Mundaneum, an attempt begun in 1895 to gather and systematically catalogue the world's knowledge, with the hope of bringing about world peace. Vannevar Bush and J.C.R. Licklider are two more contributors that advanced this idea into newer technology. Bush was seen as a researcher that assisted in making the bomb that was dropped on Hiroshima. After seeing the disaster, he wanted to create a machine that would show how technology can lead to understanding instead of destruction. This machine would include a desk with two screens, switches and buttons, and a keyboard. He named this the "Memex." This way individuals would be able to access stored books and files at a rapid speed. In 1956, Ford Foundation funded Licklider to analyze how libraries could be improved with technology. Almost a decade later, his book entitled "Libraries of the Future" included his vision. He wanted to create a system that would use computers and networks so human knowledge would be accessible for human needs and feedback would be automatic for machine purposes. This system contained three components, the corpus of knowledge, the question, and the answer. Licklider called it a procognitive system.
Early projects centered on the creation of an electronic card catalogue known as Online Public Access Catalog (OPAC). By the 1980s, the success of these endeavors resulted in OPAC replacing the traditional card catalog in many academic, public and special libraries. This permitted libraries to undertake additional rewarding co-operative efforts to support resource sharing and expand access to library materials beyond an individual library.
An early example of a digital library is the Education Resources Information Center (ERIC), a database of education citations and abstracts, which was created in 1964 and made available online through DIALOG in 1969.
In 1994, digital libraries became popular due to the $24.4 million that the U.S. federal funded among six universities for research. The universities included Carnegie Mellon University, University of California-Berkeley, University of Michigan, University of Illinois, University of California-Santa Barbara, and the Stanford University.