Et al. means and others and there are times when you must use it in your in-text references (APA) or footnotes (Chicago).
Chicago Style:
For works by three or more persons, in a footnote, only the name of the first author is included, followed by et al. with no intervening comma.
APA Style:
For works by three or more persons, give the name of the first named author only followed by "et al." for in-text citations.
There are multiple examples in the Referencing Guides - see the Author section in both the Journals and Books pages.
