Two places where you can look to find an article's DOI:

• The front page of the journal article.  The DOI may be found on the upper left, upper right, at the bottom of the front page of the journal.
• The landing page of a particular issue of a journal. Google the title of the journal. At the journal's website, you may be able to look for a particular page that features a specific volume and issue of that journal. The page that features information on each article in a particular issue is sometimes known as the landing page.

The free DOI lookup at crossref.org is an easy-to-use resource for finding a DOI.

Use this lookup page if you have the title of an article and an author name, and you want to find its DOI.

However, note that not all articles have DOIs.

Not all articles have DOIs.

• Older articles (many published pre-2000s) don't.
• Print-only articles don't. The DOI concept only deals with electronic resources.

If for these reasons or some other reason you can't find an article's DOI, you can do one of the following things, depending on what information you have or are able to find:

• Cite the journal's homepage URL if there is no DOI assigned and the reference was retrieved online.
• Cite a stable/permanent link that's provided by the database in which the journal article is located.