Open access (OA) refers to online research outputs that are free of all restrictions on access (e.g. access tolls) and free of many restrictions on use (e.g. certain copyright and license restrictions). Open access can be applied to all forms of published research output, including peer-reviewed and non peer-reviewed academic journal articles, conference papers, theses, book chapters, and monographs. Two degrees of open access can be distinguished: gratis open access, which is online access free of charge, and libre open access, which is online access free of charge plus various additional usage rights. These additional usage rights are often granted through the use of various specific Creative Commons licenses. Libre open access is equivalent to the definition of open access in the Budapest Open Access Initiative, the Bethesda Statement on Open Access Publishing and the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities. There are multiple ways authors can provide open access to their work. One way is to publish it and then self-archive it in a repository where it can be accessed for free, such as their institutional repository, or a central repository such as PubMed Central. This is known as ‘green’ open access. Some publishers require delays, or an embargo, on when a research output in a repository may be made open access. Several initiatives provide an alternative to the American and English language dominance of existing publication indexing systems, including Index Copernicus, SciELO and Redalyc. A second way authors can make their work open access is by publishing it in such a way that makes their research output immediately available from the publisher. This is known as ‘gold’ open access, and within the sciences this often takes the form of publishing an article in either an open access journal, or a hybrid open access journal. The latter is a journal whose business model is at least partially based on subscriptions, and only provide Gold open access for those individual articles for which their authors (or their author’s institution or funder) pay a specific fee for publication, often referred to as an article processing charge. Pure open access journals do not charge subscription fees, and may have one of a variety of business models. Many, however, do charge an article processing fee. Widespread public access to the World Wide Web in the late 1990s and early 2000s fueled the open access movement, and prompted both the green open access way (self-archiving of non-open access journal articles) and the creation of open access journals (gold way). Conventional non-open access journals cover publishing costs through access tolls such as subscriptions, site licenses or pay-per-view charges. Some non-open access journals provide open access after an embargo period of 6–12 months or longer (see delayed open access journals). Active debate over the economics and reliability of various ways of providing open access continues among researchers, academics, librarians, university administrators, funding agencies, government officials, commercial publishers, editorial staff and society publishers.