Interlibrary loan (abbreviated ILL, and sometimes called interloan, interlending, document delivery, or document supply) is a service whereby a patron of one library can borrow books, DVDs, music, etc. and/or receive photocopies of documents that are owned by another library. The user makes a request with their home library; which, acting as an intermediary, identifies libraries with the desired item, places the request, receives the item, makes it available to the user, as well as arranges for its return. The lending library usually sets a due date and overdue fees of the material borrowed. Although books and journal articles are the most frequently requested items, some libraries will lend audio recordings, video recordings, maps, sheet music, and microforms of all kinds. In some cases, nominal fees accompany the interlibrary loan services.
The term document delivery may also be used for a related service, namely the supply of journal articles and other copies on a personalized basis, whether these come from other libraries or direct from the publishers. The end user is usually responsible for any fees, such as costs for postage or photocopying. Commercial document delivery services will borrow on behalf of any customer willing to pay for their rates.
Interlibrary loan, or resource sharing, has two operations: borrowing and lending.
- A borrowing library sends an owning library a request to borrow, photocopy, or scan materials that is needed by their patron.
- The owning library fills the request by sending materials to the borrowing library or supply a reason for why the request cannot be filled.
- If the item is sent, the borrowing library notifies the patron when the item arrives.
Interlibrary loan and resource sharing have a variety of systems and workflows, often based on the scale of service, regional networks, and library systems. Processes are automated by computer systems such as VDX based on ISO ILL standards 10161 and 10160. Two major systems are used heavily: ILLiad developed by Atlas Systems and Worldshare Management System by OCLC. In 2017, OCLC announced a new interlibrary loan management system called Tipasa, which is built on the OCLC WorldShare technology platform, and is the first entirely cloud-based interlibrary loan management system. 
Loan requests between branch libraries in the same local library system are usually filled promptly, while loan requests between library systems may take weeks to complete. However, if an item is rare, fragile, or exceptionally valuable, the owning library is under no obligation to release it for interlibrary loan. Some collections and volumes, especially bound journals and one-of-a-kind manuscripts, are non-circulating, meaning that they may not be borrowed. Books may be delivered by mail or by courier service. Photocopies may be faxed, or scanned and delivered electronically. Urgent requests are placed if the item is needed right away; sometimes for additional fees. Public libraries do not usually offer urgent service.
Interlibrary loan provides users with access to articles from journals that their library does not have in its collection or is subscribed to. In the United States, most libraries follow guidelines established by the Commission on New Technological Uses of Copyrighted works (CONTU), which established that libraries should pay publishers' fees if more than 5 ILL requests are filled from within the past 5 years from a specific publication. This guideline is referred to in United States Libraries as the "Rule of Five."
In addition, many journal or database licenses specify whether a library can or cannot supply journal articles via ILL, with many libraries taking an approach to negotiate for ILL to be allowed in licenses. When licensed to send articles via Interlibrary Loan, and having examined the need to pay copyright fees for articles, article processing has become highly automated in Interlibrary Loan. In the early 1990s the Research Library Group (RLG) created and released Ariel, a software that made communicating both photocopies and native digital articles more efficient. In the early 2000s Atlas Systems, creators of the ILLiad software system, created Odyssey, which allowed for direct communication of articles between libraries, and ultimately direct sending of articles to library patrons. Although Odyssey usage and features increased quickly, OCLC realized an important need among its member libraries, and created Article Exchange, which is a cloud-based secure article sharing platform that automatically deletes articles after a specified number of downloads and/or a number of days.
As many libraries shifted their journal subscriptions to digital, and citation information became much more available with tools such as Google Scholar, Interlibrary Loan of articles has effectively become a large part of Interlibrary Loan services.
In the United States
In 1886 U. L. Rowell, Librarian at the University of California, Berkeley, sought permission to begin Interlibrary Loan; his request was granted during the years 1894–1898.
In 1894 Rowell initiated U.C. Berkeley's first program of interlibrary lending, with the California State Library as partner. Later that year Rowell expanded the invitation for a group of libraries, such as NUCMC. Librarians then filled out a standardized form (i.e. an ALA Interlibrary Loan Request Form 2002) and sent it by postal mail to a library that owned a copy. This procedure is still used by the few libraries that are not members of an electronic interlibrary loan network.
In 1994, the Reference and User Services Association (RUSA) of the ALA (America Library Association) formed an ALA Interlibrary Loan Code for the United States, which sought to establish resource sharing as a core service and to provide guidelines for libraries. The RUSA section on Resource Sharing has also engaged in initiatives to expand resource sharing, including the Rethinking Resource Sharing Initiative and Committee.
Since the mid-1980s, searching for books located at other libraries has become easier, as many libraries have enabled their users to search their online catalogs at the library or over the Internet. Today, everyone can freely use WorldCat.org to identify which needed items that are not owned by their local libraries. Medical libraries primarily use DOCLINE, developed by the National Library of Medicine, which comprises libraries in the National Network of Libraries of Medicine.
The Ohio State University and others in Ohio began integrating campus library systems at an early date. In the 1960s, state funds supported the development of the Online Computer Library Center (at that time called the Ohio College Library Center). OCLC has since grown into an international organization with a database of 30 million entries representing materials held in more than 10,000 libraries.
Link+ is an interlibrary loan scheme in California and Nevada, and OhioLINK is the system used in Ohio, where the catalogs and databases of the state's libraries are joined electronically.
Resource sharing networks
Libraries have established voluntary associations, often on a regional basis, to provide an online union catalog of all the items held by all member libraries. Whenever a library adds a new title to its catalog, a copy of the record is also added to the union list. This allows librarians to quickly determine which of the other libraries hold an item. Software then facilitates the request and supply tasks. In the U.S., Online Computer Library Center (OCLC) is used by public and academic libraries. Formerly, another network RLIN (Research Libraries Information Network) was used primarily by academic libraries but merged with OCLC on October 1, 2007. Australia and New Zealand use Libraries Australia and New Zealand Libraries' Catalogue respectively, the national bibliographic networks of those countries.
Online requests are usually submitted via OCLC's WorldCat or FirstSearch in the United States. Libraries without access to either can participate in interlibrary loan by submitting requests by postal mail, fax, email, or telephone call. These are referred to as manual requests. Manual requests can be submitted in the United States through the American Library Association. Some libraries establish reciprocal arrangements with each other to supply loans and copies for free. Examples of such arrangements in the United States include Libraries Very Interested in Sharing (LVIS), Amigos, Mid-America Association of Law Libraries (MAALL), Bibliographical Center for Research, and the Greater Western Library Alliance (formerly the Big 12 Plus Library Consortium). Sometimes these arrangements include other services such as the Trans-Amigos Express (TAE) courier services, which will ship and deliver items to Amigos members on the TAE route. Individual libraries can agree to reciprocal arrangements between each other.
- "ILLiad: Atlas Systems". www.atlas-sys.com. Retrieved 2016-03-24.
- author-replicator. "WorldShare Management Services". www.oclc.org. Retrieved 2016-03-24.
- David, Free (April 2017). "OCLC introduces Tipasa interlibrary loan management system". College & Research Libraries News. 78 (4): 186–187. doi:10.5860/crln.78.4.9646. ISSN 0099-0086 – via Highwire Press Free.
- Croft, Janet Brennan (2005-05-31). "Interlibrary Loan and Licensing". Journal of Library Administration. 42 (3-4): 41–53. doi:10.1300/J111v42n03_03. ISSN 0193-0826.
- "Article Exchange".
- "ALA Interlibrary Loan Code".
- "Rethinking Resource Sharing".
- "RUSA Stars Rethinking Resource Sharing".
- "DOCLINE® System". www.nlm.nih.gov. Retrieved 2016-03-24.
- Aggarwal, Anil (2000). Web-Based Learning and Teaching Technologies: Opportunities and Challenges. Idea Group Inc. p. 21. ISBN 978-1-878289-60-5. OCLC 43095789.
- "Link+ Catalog". csul.iii.com. 2012. Retrieved 4 March 2012.
- "What Is OhioLINK". ohiolink.edu. 2012. Archived from the original on February 6, 2012. Retrieved 4 March 2012.
- New Zealand Libraries' Catalogue. National Library of New Zealand.
- "Libraries Very Interested in Sharing". cyberdriveillinois.com.
- "Amigos Library Services - Resource Sharing Through Technology". amigos.org.
- "Mid-America Association of Law Libraries". Aallnet.org. Retrieved 2013-12-31.
- "GWLA". GWLA. Retrieved 2013-12-31.
- "Trans Amigos Express (TAE)". amigos.org.
- Boucher, Virginia. Interlibrary Loan Practices Handbook, 2nd Edition; American Library Association, Chicago and London, 1997
- Linda Frederiksen, Joel Cummings, Lara Cummings & Diane Carroll. (2011). Ebooks and Interlibrary Loan: Licensed to Fill? Journal of Interlibrary Loan,Document Delivery & Electronic Reserve. 21 (3), 117–131. http://www.tandfonline.com/doi/abs/10.1080/1072303X.2011.585102#
- Levine-Clark, Michael. (2011) Whither ILL? Wither ILL: The Changing Nature of Resource Sharing for E-Books. Collaborative Librarianship 3(2):71-72
- Newcombe, Luxmoore. Library Co-operation in the British Isles, in series, Practical Library Handbooks, no. 4. London: G. Allen & Unwin, 1937.
- 2008 Document Delivery - Best Practices and Vendor Scorecard—Outsell, Inc.
- Journal of Library Administration; Volume 23, Numbers 1/2, 1996
The leading journals in the field of interlibrary loan are:
- Interlending and Document Supply
- Journal of Access Services
- Journal of Interlibrary Loan, Document Delivery and Electronic Reserve, Haworth Press. (Earlier title: Journal of Interlibrary Loan, Document Delivery & Information Supply).
- Department of Education and Science (1962) Inter-library Co-operation in England and Wales: report of the working party appointed by the Minister of Education in March 1961. (Chairman: E. B. H. Baker.) London: H. M. S. O.