A digital object identifier (DOI) is a character string used to identify and provide a permanent link to electronic documents. The new APA guidelines for citing electronic resources call for the inclusion of the DOI in the citation whenever possible. The DOI can often be found on the first page of scholarly journal articles as well as in the database record for that article.
Please bear in mind that some materials (especially older ones) may not have a DOI. See the attached flowchart from the American Psychological Association for guidance on citing electronic materials for which no DOI is found.
You can find instructions for adding the DOI to your APA citations in the link below.