DOI stands for "Digital Object Identifier" and it is a unique identifier made up of numbers and letters that provides a persistent link to an article's location on the Internet. Oftentimes, a DOI leads to content that's available electronically. When you have a DOI, you don’t need a full citation to track a source down. Just plug the DOI into a DOI resolver at the International DOI Foundation or at a DOI registration agency like CrossRef and you'll jump straight to a metadata page with the full citation and a link to full text if available.