APA style of citing requires the use of DOI's (Digital Object Identifiers) for all resources available online. Many databases will provide the DOI for you along with the rest of an article's citation information (title, author, etc.).  However, if you can not find a DOI for your article, or if you have a DOI, but don't know what article it is for, use the following online DOI Resolver:     www.crossref.org