DOI is an abbreviation for Digital Object Identifier. It is an alphanumeric code assigned to specific articles and other electronic publications. An analogy for a DOI is a cell phone number; if you call that number you can get that publication no matter where it is located on the internet. Also, like a cell phone number, DOI did not exist years ago but may be obtained later OR the particular publication may not have one. The difficulty is finding the DOI if it is not printed on the publication but was obtained after publication.