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Open access (OA) or open access publishing is the publication of material in such a way that it is available to all potential users without financial or other barriers. An open access publisher is a publisher producing such material. Many types of material can be published in this manner: scholarly journals, known specifically as open access journals, magazines and newsletters, e-text or other e-books (whether scholarly, literary, or recreational), music, fine arts, or any product of intellectual activity. In this context, non-open access distribution is called "toll access" or "subscription access".

Open access can be provided by traditionally-organized publishers, or under other arrangements. With respect to scholarly material, some distribution is carried out by locally organized and subsidized publishers; an example is the production of Annals of Mathematics, produced and supported by the Princeton University Department of Mathematics and the Institute for Advanced Study. Other publishing platforms include twidox which is a free, user generated online library of "quality" documents that allows individuals and organisations to easily publish, share and search for them. More normally it is a specialized publisher. Some open access publishers publish only open access material, such as PLoS; some publish open access journals as well as subscription-based material, such as BioMed Central (BMC).

The term has also been used in a wider sense to include publishers of Hybrid open access journals, which provide open access only for some articles, those for which payment is made on behalf of the author. It can similarly be used for publishers of Delayed open access journals, in which the articles are open access only after a period of embargo. Even more loosely, the term is also used to describe publishers that permit or encourage self-archiving by authors and institutions.

The term is most often used in reference to academic journals, where there is active debate on the appropriate distribution model. Most open access material in this context is distributed via the World Wide Web. OA articles usually have limited copyright and licensing restrictions.

The first major international statement on open access was the Budapest Open Access Initiative in February 2002. This provided a definition of open access, and has a growing list of signatories. Two further statements followed: the Bethesda Statement on Open Access Publishing in June 2003 and the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities in October 2003.

OA has since become the subject of much discussion amongst researchers, academics, librarians, university administrators, funding agencies, government officials, commercial publishers, and society publishers. Although there is substantial (though not universal) agreement on the concept of OA itself, there is considerable debate and discussion about the economics of funding peer review in open access publishing, and the reliability and economic effects of self-archiving.

There are about 20-25,000 peer-reviewed journals in all across all disciplines, countries and languages. About 10 - 15% of them are OA journals, as indexed by the Directory of Open Access Journals (gold OA). Of the more than 10,000 peer-reviewed non-OA journals indexed in the Romeo directory of publisher policies (which includes most of the journals indexed by Thomson/ISI), over 90% endorse some form of author self-archiving (green OA): 62% endorse self-archiving the author's final peer-reviewed draft or "postprint," 29% the pre-refereeing "preprint."