Interlibrary Loan: How It Works and the Systems Behind It

To a patron, an interlibrary loan is one click and then a wait. To the two libraries involved, it is a request that has to be verified, matched against holdings across a whole network, routed to a library willing and able to supply it, shipped, checked out on a system that did not create the record, and tracked home again. Almost everything a user experiences as “slow” or “fast” is decided in that middle stretch, and that is the part nobody writes about.

An interlibrary loan is a transaction in which one library borrows an item, or obtains a copy of one, from another library on behalf of a user. The requesting library places and receives the request. The supplying library fills it. Both operate under shared codes, agreements and integrated library systems.

If you are trying to request something for yourself, start at your own library’s website, because requests are placed through the library that issued your card. The rest of this article is for the people on the other side of that form: library staff, interlibrary loan coordinators, and the consortium and state library teams who have to make the network perform. It covers the request lifecycle, realistic turnaround, the copyright and licensing limits on what can be supplied, and what the software layer contributes that a local integrated library system cannot.

What is an interlibrary loan?

Interlibrary loan, usually shortened to ILL, is the mechanism that lets a library serve a request for something it does not own. Every participating library plays two roles, often on the same day: as a requesting library it asks others to supply material for its users, and as a supplying library it fills requests coming the other way, which is where the term interlibrary lending comes from.

What makes this work at a national scale is that it is treated as an obligation rather than a courtesy. The Interlibrary Loan Code for the United States, approved by the RUSA Board in June 2023 in a revision that merged the code and its former explanatory supplement into a single document, sets out responsibilities on both sides. Requesting libraries should seek material through interlibrary loan when their own collections cannot meet a user’s need. Supplying libraries should participate in sharing, on a principle the code states directly: “The effectiveness of the national interlibrary loan system depends upon participation of libraries of all types and sizes.”

That framing matters operationally. A library that borrows heavily and lends reluctantly is not being efficient, it is drawing down a shared capacity that depends on participants honoring both halves of the arrangement.

Interlibrary loan, document delivery and consortial borrowing: what is the difference?

These terms get used interchangeably in conversation and mean materially different things in a workflow. Getting them straight matters when you are writing policy, reporting statistics, or specifying a system.

Interlibrary loan is the umbrella here, not one of four parallel options. It covers both the loan of a returnable item and the supply of a copy the user keeps.

Within interlibrary loan itself, the distinction that drives everything operationally is returnable versus non-returnable. A returnable creates a circulation obligation, a due date, a return shipment and a risk of loss. A non-returnable ends when it is delivered. That single difference drives most of the cost, speed and policy variation across library resource sharing, and it explains why article requests behave so differently from book requests.

How does an interlibrary loan work, from request to return?

The sequence below reflects how mediated and unmediated requests typically move through a resource sharing network. It is not a universal procedure. Consortia, state systems and platforms differ substantially, particularly at stages three and four, and a small reciprocal group will handle several of these steps by agreement rather than by software.

1. The request is placed: A user submits an interlibrary loan request through the catalog, a discovery interface, or a request form, or staff place it on their behalf. In patron-initiated workflows, this shifts staff effort away from initial data entry and toward exception handling.
2. The request is verified: Staff or the system confirm the citation and that the item is genuinely not held locally. Incomplete or incorrect citations are a routine cause of delay, because a request that cannot be matched to a bibliographic record cannot be matched to a lender either.
3. Candidate lenders are identified: The request is searched against a union catalog to find libraries holding the item. Common US examples include OCLC’s WorldCat network and, for health sciences libraries, the National Library of Medicine’s DOCLINE, alongside the union catalogs many states and consortia run themselves. That catalog can be a maintained central index of member holdings, or a virtual one built on the fly by querying member catalogs in real time using Z39.50. The difference is not academic: a maintained index is fast but only as current as its last update, while a live search is current but depends on every member system responding.
4. A lender list is built and the request is routed: The system ranks candidate lenders and sends the request to the first. Two behaviors do most of the work. Load leveling distributes requests so the largest and best-stocked libraries do not absorb a disproportionate share of regional lending. Deflection passes a request the current lender cannot fill straight to the next one rather than bouncing it back to the start, which is the difference between a request that quietly moves on and one that sits unfilled for a week.
5. The item is supplied and delivered: Returnables travel by courier or mail. Non-returnables are delivered electronically. For physical material, courier frequency and routing usually set the floor on turnaround, not the software, which is why two libraries on the same platform can offer very different service levels.
6. The item is checked out, used and returned: The borrowing library’s circulation system takes over, so the item is issued, renewed and discharged like anything else on the shelf. NCIP and SIP2 let a resource sharing platform hand an item into that local system without staff rekeying it. Where the link is missing, libraries end up keeping a parallel manual record for every loan they receive.

How long does an interlibrary loan take?

It depends almost entirely on whether the item has to be shipped. Articles and chapters can arrive within hours or days because supply is electronic. Physical items are slower, and published service expectations reflect it: East Carolina University’s interlibrary loan FAQ tells users that physical items can sometimes arrive within five days, while shipping alone commonly runs from three to fourteen days depending on the carrier.

For anyone managing the service, the variance matters more than the average. What drives it:

• Lender list depth, and whether deflection is automatic: A request with three candidate lenders runs out of options far sooner than one with twenty, and manual reprocessing of an unfilled request adds days on its own.
• Demand on the title: New and popular items are often already on hold at the libraries that own them.
• Courier frequency: Identical software will produce very different service levels on a network with daily courier runs and one with twice-weekly runs.
• Whether the request crossed a boundary: In-state and in-consortium requests are usually faster because the agreements, routing and delivery are already established.

Reported as a single average, all of that variation cancels out and hides the problem. Splitting returnables from non-returnables, and in-network from out-of-network, is normally the first change that makes interlibrary loan statistics useful to anyone making a decision.

What does interlibrary loan cost, and what does copyright allow?

For public library users, interlibrary loan is often free or low cost. The costs are real but sit with the library: lending charges from some suppliers, courier and postage, staffing, and the platform. Reciprocal and statewide agreements commonly remove per-transaction charges between members, which is one reason libraries join them.

Three separate layers govern what can actually be supplied, and conflating them causes most of the confusion.

Section 108 of Title 17 of the United States Code provides the statutory basis for libraries to reproduce and distribute copies in support of interlibrary loan, subject to conditions including that the copy becomes the property of the user and that the library displays the required copyright warning.

The CONTU guidelines, widely known as the “suggestion of five,” are a working convention many US libraries apply to periodical article requests. They were developed to interpret the limits in Section 108(g)(2) and are guidelines rather than statute, so they usually function as a policy threshold. That distinction matters when someone asks whether a sixth request is illegal. It is not, but it may be outside your policy.

Licensing is a separate constraint, and the one most easily overlooked. Electronic resource agreements can restrict or prohibit interlibrary loan supply independently of copyright law, and terms vary by publisher and by contract. A library can be legally entitled to supply a copy under Section 108 and still be unable to supply it from its licensed electronic version. Requests are declined for many reasons, including items in use, non-circulating or fragile material, cost and unverifiable citations, but a license restriction is the one most likely to catch both staff and users out, because nothing about the library’s holdings signals that the item cannot be supplied.

What does interlibrary loan software actually do?

An integrated library system manages what a library owns. Interlibrary loan software manages requests for what it does not own. That second job has to work out who holds an item across many separate catalogs, decide who to ask and in what order, handle refusals without human intervention, authenticate a user against a system it does not control, and hand the result into someone else’s circulation module. It is library automation extended across institutional boundaries rather than applied inside one, and the boundary is what makes it hard.

SHAREit, the resource sharing component of the Soutron library information platform, is used across 16 US states, among them Texas, Illinois, Pennsylvania, Tennessee and Kansas, and the Canadian province of British Columbia. It builds a dynamic union catalog through Z39.50 searching, offers multiple levels of lender lists with eight load-leveling options and unlimited lenders per request, deflects unfillable requests automatically, and authenticates remote patrons against their own library’s patron file. It can include e-resource database holdings, including OverDrive and Hoopla ebooks, and link out to those alongside physical holdings, and it handles electronic delivery of non-returnables such as journal article copies. The patron interface is built to WCAG 2.0 Level AA and Section 508, tested with JAWS, NVDA and Dragon.

It is also designed to interface with multiple ILS and interlibrary loan systems rather than assume a single-vendor network, including OCLC WorldCat, ILLiad and Tipasa. Where a library runs the VERSO integrated library system as well, the NCIP-compliant Circulation Interlibrary Loan Link embeds SHAREit inside VERSO so staff and patrons search, place and process requests in a single interface, with holds generated automatically and interlibrary loan items managed as collection holdings. VERSO supports Z39.50, NCIP and SIP2, ISO 10160 and 10161, and ISO 18626, the current international standard for interlibrary loan transactions.

What should a library or consortium ask before choosing a resource sharing platform?

These questions come from the failure points described above rather than a generic feature list. Take them into an internal requirements meeting before you take them to a vendor.

• How is the lender list built and ordered, and how much control do we have over that ordering?
• What load leveling options exist, and can an individual library cap what it absorbs?
• Is deflection automatic, and what happens to a request that no member can fill?
• How does the platform authenticate our patrons against our own system’s records?
• Which circulation protocols are supported, and what specifically is required to integrate with the systems our members already run?
• Are licensed e-resources and ebook platforms inside the same request path as print, or a separate manual workflow?
• What accessibility standard does the patron interface meet, and which assistive technologies has it been tested with?
• What reporting do we get on turnaround and fill rate, the share of requests that end in a successful supply, split by returnable and non-returnable, and can each member see its own performance?

The last one is regularly underestimated. A consortium that cannot show members their own numbers will struggle to have a productive conversation about lending imbalance, and lending imbalance is what quietly erodes participation.

Frequently asked questions

What is the purpose of interlibrary loan? 

Interlibrary loan exists so that a user’s access to material is not limited by what one library happens to own. It lets small and specialized collections serve requests far beyond their holdings, and lets large collections make their depth useful beyond their own community. The system works because participating libraries commit to lending as well as borrowing.

Do all libraries participate in interlibrary loan? 

No, participation depends on institutional policy, staffing, and whether the library belongs to a consortium, state system or cooperative that provides the infrastructure. It is often organized at network level rather than library by library, so a library’s borrowing and lending reach is defined by the systems and agreements it has joined.

Can you request an interlibrary loan from another state? 

Yes, in many statewide and consortial systems, requests are routed within the participating network first, because delivery is faster and member agreements often remove transaction fees. If no library in that network can supply the item, the request moves outward to national networks. Where routing works that way, an out-of-state fill usually takes noticeably longer than an in-state one.

Why was my interlibrary loan request canceled or denied? 

Common reasons: no participating library holds the item or all copies are in use, licensing terms on an electronic version prohibit sharing, the item is too new, fragile or valuable to travel, or the citation could not be verified against a bibliographic record. A denial is rarely a judgment about the request itself.

Can ebooks and digital articles be borrowed through interlibrary loan? 

Articles and book chapters are supplied routinely as non-returnable copies, within copyright and licensing limits. Commercial ebooks are often unavailable for interlibrary lending because publisher and platform license terms may restrict or prohibit it. Whether a specific ebook can be supplied therefore depends on the applicable license rather than on whether the library has access to it, which is why an ebook clearly available to a library’s own users may still be impossible to share.

Where this leaves you

The patron-facing part of interlibrary loan has barely changed in a generation. What determines whether the service is good is the network underneath: how deep the lender lists are, whether requests deflect or stall, whether items land in circulation cleanly, and whether anyone can see the fill rate by member. Those are systems questions before they are staffing questions, and treating a turnaround problem as a workload problem is how libraries add hours to a process that needed its routing fixed.

A network that is growing, adding member types, or running across more than one underlying system usually reaches a point where the platform stops being a background utility and starts setting the ceiling on the service. That is the moment to look properly at what SHAREit does with routing and interoperability.