A DOI, or digital object identifier, is a unique combination of numbers and letters that identifies and permanently links to specific articles or internet content. The publisher of the content assigns and makes the doi available for electronically published content.
What does a doi look like in a citation?
In APA Format:
Orr, L., & Govindjee, G. (2010). Photosynthesis online. Photosynthesis Research, 105(2), 167-200.
MLA does not require a DOI at this time, as of the 7th edition. "A digital object identifier (DOI) is a unique alphanumeric code assigned by the International DOI Foundation to identify content and provide a persistent link to its location on the Internet. MLA style (7th ed.) does not require inclusion of DOIs. (Source: MLA handbook for writers of Research papers, 7th ed.) "