Last Updated: Oct 27, 2015 Views: 43
Not every article will have a DOI. DOI (Digital Object Identifier) or other unique identifiers assigned to articles have only come into use relatively recently. Articles written prior to 1998 will not have a DOI. Articles written after 1998 may or may not have one, but DOIs have come into wide spread use within the last 5 to 10 years. So the more recent the article the more likely it will have a DOI.
Some databases such as PubMed now include the DOI in the citation to the article. There is also a free DOI lookup tool: