A DOI (Digital Object Identifier) is a standardized method for identifying and locating content on the Internet. A DOI looks something like this:
DOIs are most commonly found with articles, but other types of online sources, such as ebooks or ebook chapters, can also have DOIs.
DOIs often appear in a citation, in the abstract of an article, or at the top or bottom of the first page of the PDF of an article.
You may need to include a DOI for some citation styles. For example, Chicago and APA styles require a DOI for online journal articles. If you cannot find the DOI of an article, try CrossRef's free DOI lookup tool.