Q. What is a DOI number? How do I use it in a citation?

A DOI (Digital Object Identifier) is a standardized unique number given to many (but not all) articles, papers, & books, by some publishers, to identify a particular publication. 

To find a DOI number for an article, look on the first page and in the header or footer information.  If you see a "DOI:" followed by a string of numbers, you'll know that you have found it. 

If you have a reference and can't find the DOI number, or have a DOI number and are missing the reference, click here to search Crossref. 

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To use a DOI in a citation:

All the style guides used by APUS state that, when citing electronic sources, one should use the most stable link available. If you cannot locate a DOI for a source you want to cite -- simply leave it out.

• APA: APA citation style calls for the inclusion of DOI numbers in a citation - when available. Read more here.
   
• Bluebook: The Bluebook editors have no designated preference re: DOIs. They do state that “All efforts should be made to cite the most stable electronic location available,” (Section 18.2.2 ) but this may or may not be a DOI.
   
• Chicago: The CMS editors say “A DOI, if it is available, is preferable to a URL.” (Section 14.184 )
   

• MLA: The MLA Handbook says: “When possible, citing a DOI is preferable to citing a URL.” (48). Read more here.
   
• Turabian: The editors of Turabian state “If a website gives a preferred form of the URL, along with the citation for a source, use that rather than the URL in your browser’s address bar. Some sources are identified by a DOI (digital object identifier). URLs based on DOIs are more persistent and stable that URLs. To cite a source that includes a DOI, append the DOI to https://dx.doi.org/ in your citation.” (Section 15.4.1.3)

See also: 

• Can I use a DOI number to find an article?