The LOCKSS ("Lots of Copies Keep Stuff Safe") project, under the auspices of Stanford University, is a peer-to-peer network that develops and supports an open source system allowing libraries to collect, preserve and provide their readers with access to material published on the Web. Its main goal is digital preservation. The system attempts to replicate the way libraries do this for material published on paper. It was originally designed for scholarly journals, but is now also used for a range of other materials. Examples include the SOLINET project to preserve theses and dissertations at eight universities, US government documents, and the MetaArchive Cooperative program preserving at-risk digital archival collections, including Electronic Theses and Dissertations (ETDs), newspapers, photograph collections, and audio-visual collections. A similar project called CLOCKSS (Controlled LOCKSS) "is a tax-exempt, 501(c)3, not-for-profit organization, governed by a Board of Directors made up of librarians and publishers." CLOCKSS runs on LOCKSS technology. Problem Traditionally, academic libraries have retained issues of scholarly journals, either individually or collaboratively, providing their readers access to the content received even after the publisher has ceased or the subscription has been canceled. In the digital age, libraries often subscribe to journals that are only available digitally over the Internet. Although convenient for patron access, the model for digital subscriptions does not allow the libraries to retain a copy of the journal. If the publisher ceases to publish, or the library cancels the subscription, or if the publisher's website is down for the day, the content that has been paid for is no longer available. Methods The LOCKSS system allows a library, with permission from the publisher, to collect, preserve and disseminate to its patrons a copy of the materials to which it has subscribed as well as open access material (perhaps published under a Creative Commons license). Each library's system collects a copy using a specialized web crawler that verifies that the publisher has granted suitable permission. The system is format-agnostic, collecting whatever formats the publisher delivers via HTTP. Libraries which have collected the same material cooperate in a peer-to-peer network to ensure its preservation. Peers in the network vote on cryptographic hash functions of preserved content and a nonce; a peer that is outvoted regards its copy as damaged and repairs it from the publisher or other peers. The LOCKSS license used by most publishers allows a library's readers access to its own copy, but does