A DOI (digital object identifier) is a unique alphanumeric string that provides a persistent link to content online. In APA style, try to include a DOI for every reference if possible.
The current APA manual calls for the DOI number for all digital documents if present. But what if there is no DOI? Then you might need to use the URL of the journal's home page OR the URL of the subscription database in which you found the article. Confused? This flow chart from APA is a useful way to determine what you need in your citation.
This guide was created with thanks to the Librarians at University of South Carolina's Thomas Cooper Library.