What is a DOI?
A Digital Object Identifier or DOI is an unique code that is used to identify materials that exist in an online environment. They are very similiar to the ISBN codes that are used to identify books and magazines. By using this code, students, scholars, editors and librarians can locate an online article even if it it's URL (web address) changes over time. The DOI for an article will never change, providing a permanent way to identify articles online.
The location of a DOI will be slightly different for each publication. Many articles, particularly older works, may not have DOI. Look for the DOI letters followed by an alphanumeric sequence that begins with the number 10. They are usually at the top of an article, nearby or long with the other citation information.