A DOI (Digital Object Identifier) is a series of numbers used to identify journal articles. Every article has its own unique DOI. The DOI of an article can often be found within the article itself, usually on the first or last page. Check out our referencing page for more information on DOIs and to see if/how they are used in the referencing system you are using.
While the DOI will often be found on the first or last page of an online article, you may also use the following free online tool by conducting a 'metadata search': http://search.crossref.org/