Which children's book series by Dav Pilkey has been frequently challenged for inappropriate humor?

The children's book series by Dav Pilkey frequently challenged for inappropriate humor is Captain Underpants.

The series began in 1997 and follows fourth-graders George Beard and Harold Hutchins, whose hypnotized principal becomes the underwear-wearing superhero Captain Underpants. Its deliberately silly blend of potty jokes, comic-book violence, pranks, and disrespect for authority made it especially popular with children—and controversial with some adults.

The American Library Association reported that Captain Underpants was the most challenged book in the United States in both 2012 and 2013. Complaints commonly cited offensive language, humor considered unsuitable for the intended age group, disruptive behavior, and disobedience toward authority figures. Some challenges also involved the final book's portrayal of a same-sex couple.

A common mix-up is confusing the series with Pilkey's spin-off, The Adventures of Super Diaper Baby. That title has also appeared on challenged-book lists, but Captain Underpants is the canonical answer to this question.