A DOI (digital object identifier) is a unique alphanumeric string assigned to a specific electronic resource which provides a persistent link to its location.
ex: doi: 10.1037/0278-6126.96.36.1999
When available, include the DOI in your journal article references.
See pages 187-192 & 198-199 of the APA Publication Manual for more information
As indicated in the Publication Manual of the American Psychological Association (pg 198):
"If no DOI is assigned to the content and you retrieved it online, include the home page URL for the journal, newsletter, or magazine in the reference. Use this format: Retrieved from http://www.xxxxxxxx"
Databases: Most databases include a DOI (if available) in the full record. For example, in PsycINFO, choose the Complete Reference link to see if a DOI is listed. See an example here.
E-Journals: You can often retrieve the article's DOI from the publisher's website for your particular article. See an example here.
Print Journals: Look on the first page of the journal to see if a DOI is listed (top corner, bottom of the page, etc.). See an example here.