MLA 9 Style Rule for DOI
When citing journal articles (print or electronic), MLA 9 style requires the inclusion of the DOI, if it is available (see example below).
What it is:
A DOI is a unique "digital object identifier" that is permanently attached to a specific article. It is a cross of numbers and letters, and always begins with a "10."
When & How to Cite a DOI:
If a DOI is available for an article being cited, it must be included (as the final element, without a full stop/period) in the reference.
When used, the DOI replaces an article's URL in the reference.
Neither the URL nor an access date are included when the DOI is used in a reference.
Locating the DOI
You can find the DOI either...
1) in the database record (there will be a field in the article record that says "DOI") or
2) on the first page of the article, usually near the copyright information.
No Apparent DOI?
If you don't see a DOI in the online article information or printout, you can query it's DOI status by clicking http://www.crossref.org/guestquery/ and entering in your citation information.
Further Info
For more information, consult the MLA 9 Style Manual. The APA website also has a helpful video tutorial on finding DOIs.