A Digital Object Identifier (DOI or doi) creates a permanent link for an item. It allows a Uniform Resource Locator (URL or web address) to be defined for each journal article, or other online material, including datasets. It is a simple mapping between the DOI and URL. It prevents the "Link not found" errors on the web. All trusted scholarly journals assign unique DOIs to individual published journal articles, and registers the DOIs with a DOI system. An article can only be assigned one DOI, and that DOI will remain persistent in the future – regardless of changes in editor, publisher or website host.

A requirement of the DOI system is that references cited should, wherever possible, contain a DOI (outgoing DOIs). Doing this makes it easier to generate citation counts.

A DOI expands to a URL by adding http://dx.doi.org/ before the prefix. The original standard for displaying a DOI was without this component but the current DOI display guidelines require that a DOI be displayed as a full URL.

The steps below describe how to obtain DOIs. If you work directly with a registration agency such as Crossref, they charge an annual fee to a publisher, a fee for creating DOIs for old issues (more than two years old) and a higher fee for current issues. Approximate fees are $1 per new DOI. Updates to the URL to which the DOI refers are free. You can find out more about Crossref fees here. If your organization publishes more than one journal, you only need to register once, but each journal may have a different prefix.

Note that SciELO SA journals that are not registered with Crossref can use the ASSAf prefix: 10.17159