According to the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities (2003), “Open Access contributions must satisfy two conditions:

1. The author(s) and right holder(s) of such contributions grant(s) to all users a free, irrevocable, worldwide, right of access to, and a license to copy, use, distribute, transmit and display the work publicly and to make and distribute derivative works, in any digital medium for any responsible purpose, subject to proper attribution of authorship (community standards, will continue to provide the mechanism for enforcement of proper attribution and responsible use of the published work, as they do now), as well as the right to make small numbers of printed copies for their personal use.

2. A complete version of the work and all supplemental materials, including a copy of the permission as stated above, in an appropriate standard electronic format is deposited (and thus published) in at least one online repository using suitable technical standards (such as the Open Archive definitions) that is supported and maintained by an academic institution, scholarly society, government agency, or other well established organization that seeks to enable open access, unrestricted distribution, inter operability, and long-term archiving.”

However, different types of Open Access are possible:

Diamond Open Access is a publishing model that allows free access to peer-reviewed journal articles or scholarly monographs without article processing fees (APCs) or book processing fees (BPCs).

Gold Open Access is a publication model that guarantees free access to peer-reviewed articles through Article Processing Charges (APCs) or Book Processing Charges (BPCs).

Green Open Access is a publication model in which an article or a monograph is made freely accessible on the Internet without the support of a publisher. This can take place before, during or after the official publication of the work.