Sci-Hub is a shadow library website that provides free access to millions of research papers, without regard to copyright, by bypassing publishers' paywalls in various ways. It does not provide access to books. Sci-Hub was founded in Kazakhstan by Alexandra Elbakyan in 2011, in response to the high cost of research papers behind paywalls (see Serials crisis). The site is extensively used worldwide. In September 2019, the site's operator(s) said that it served approximately 400,000 requests per day.
In addition to its intensive use, Sci-Hub stands out among other shadow libraries because of its easy use/reliability and because of an enormous size of its collection: a 2021 study estimated, that Sci-Hub provided access to 95% of all scholarly publications with issued DOI numbers, and on 15 July 2022 Sci-Hub reported that its collection comprises 88,343,822 files.
Sci-Hub and Elbakyan were sued twice for copyright infringement in the United States, in 2015 and 2017, and lost both cases by default, leading to loss of some of its Internet domain names. The site has cycled through different domain names since then.
Sci-Hub has been lauded by some in the scientific, academic, and publishing communities for providing access to knowledge generated by the scientific community, which is usually funded by taxpayers (government grants) and with zero royalties paid to the authors. Publishers have criticized it for violating copyright, reducing the revenue of publishers, and potentially being linked to activities compromising universities' network security, though the cybersecurity threat posed by Sci-Hub may have been exaggerated by publishers.
Elbakyan questioned the morality of the publishers' business and the legality of their methods in regards to the right to science and culture under Article 27 of the Universal Declaration of Human Rights, while maintaining that Sci-Hub should be "perfectly legal." Many Sci-Hub users, including Alexandra Elbakyan herself, maintain the position that Sci-Hub is a moral imperative, and if the operation of Sci-Hub contradicts the law, it is the law that should be changed rather than banning Sci-Hub.