A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when the article is published and made available electronically. The DOI can be found in the citation for the article from the database you are searching or sometimes printed on the article near the copyright information.
If no DOI has been assigned and you are accessing the periodical online, use the URL of the website from which you are retrieving the periodical. See the flowchart below for more information.