What is a DOI, and how do I find it? A Digital Object Identifier (DOI) is a unique number assigned to an article (not to books).
If an article has a DOI, it will usually be found on the first page of the article, in the citation, or on the publisher's page for the article. You can use the CrossRef Simple Text Query Form to see if an article has a DOI (email registration required.)
Why do some articles not have DOIs? Not all articles have DOIs. To get DOIs, publishers have to choose to be a member of one of the DOI registration agencies, a process that requires a membership fee, and not all publishers choose to do this. Very old publications may also have been published before the DOI system was established.
