The Budapest Open Access Initiative (BOAI) Declaration of 14 February 2002 defines open access (OA) to scientific and scholarly literature as its "free availability on the public internet permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. The only constraint on reproduction and distribution, and the only role for copyright in this domain, should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited."

The aim of OA is to make scientific and scholarly literature and materials freely accessible to all internet users - that is, to make them accessible online free of charge and as free of technical and legal barriers as possible. One essential aspect of OA is the maximisation of the dissemination of scientific and scholarly information, as called for in the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities. Drafted in October 2003, the declaration has since been signed by leading research funders, universities, and research organisations (e.g. the German Research Foundation (DFG), the Swiss National Science Foundation (SNF), the Austrian Science Fund (FWF), the Rectors' Conference of the Swiss Universities (CRUS), the German Rectors' Conference (HRK), the Max Planck Society (MPG), and CERN). For more information on the BOAI- and Berlin Declarations, see our pages on the history of OA and on position statements on OA.

Besides direct online access to scientific and scholarly information for everyone, and the attraction of the greatest possible attention to the results of scientific research, widely acknowledged arguments in favour of OA include the speed of publication and access, the possibilities for re-use (depending on the usage licence granted), and transparency and quality-assurance considerations.

Two strategies to facilitate access to scientific and scholarly literature are customarily distinguished in discussions on OA and in practice: the gold road to OA, which refers to measures that focus on primary OA publishing - at present, particularly in OA journals; and the green road, which entails providing OA to articles that have appeared in (closed-access) journals by depositing a version of the work in an OA repository.

Usage licences that regulate OA rights and specify what the public may do with the publications in question are of particular importance, especially for the gold road. If the author decides against granting blanket rights that go beyond reading the text free of charge, the far-reaching demands contained in the above-mentioned OA declarations are not, or cannot be, fulfilled. However, the granting of blanket licences by authors may collide with other obligations - for example when exclusive rights of use in a work have already been transferred to a publisher.

In recent years, the principle of OA has been firmly established in declarations of intention made by higher education institutions and research organisations; in the funding conditions of third-party funders; and, more recently, in proposed legislation. In a diverse landscape of OA journals, publishers, and repositories, even most conventional scholarly publishers now offer their authors OA options.

The OA movement is currently receiving new impetus from intensified developments in the areas of open research data and open science.