A DOI, or Digital Object Identifier, is a string of numbers, letters and symbols used to permanently identify an electronic source or book and link to it on the web. The DOI helps readers locate a document even if the physical location of the document changes. The DOI will never change.
The DOI is typically displayed prominently in the bibliographic information for a publication. Check the first page or first several pages of the document, near the copyright notice. The DOI can also be found on the database landing page for the source.
If you still can't find the DOI, you can look it up on the website CrossRef.org (use the "Search Metadata" option).
It's important to note that not all electronic materials will have a DOI. The DOI system is a fairly recent concept, so books and articles published prior to 2000 are less likely to have DOIs.
You may find DOIs formatted in various ways:
Use either DOI's or URLs in an entry in the reference list. You do not need to include both the DOI and a URL. Do not add a full stop at the end of a reference after a DOI or URL.
Often URLs can be very long and unwieldy, making referencing trickier. In these cases, you can use an online tool to shorten URLs, such as https://bitly.com/.