In the Captain Underpants series, there are numerous running gags in each book, which vary from chapter titles to catchphrases.
List of running gags
- At the end of every book (until the 5th book) Captain Underpants flies or jumps out the window.
- Whenever Captain Underpants flies off, he shouts "Tra La LAAAAA!!"
- At the end of each book, Harold says, "Oh no!", and George says "Here we go again!". In the sixth book onward, George says "Oh no!" and Harold says, "Here we go again!".
- George and Harold often break the fourth wall. The characters in the series are all well aware that they are inside of a book, frequently referencing pages, chapters, and other books in the series. On one occasion, George speaks directly to the unseen narrator, and another time he mentions the third book, "the one with the annoyingly long title". At the end of the fourth book, Harold states,"I think this is the first time one of our books had a moral!" Also, in the fifth book, a robot Harold known as "Harold 2000" kicks a ball through page 101 and page 102.
- The last phrase of a chapter is often answered by the title of the next chapter. For example, in the fourth book, Mr. Krupp says "Where, oh where?"; and the title of the next chapter is "Here, oh here".
- Every book has a chapter with just a few words and one page, titled "To Make a Long Story Short". "It did" is the usual phrase in the chapter. The eighth book bent this a little, making two chapters on it, the first being the usual, and the second being entitled "To Make An Even Longer Story Even Shorter".
- Every book begins with "George and Harold" as the first chapter, with George and Harold switching a sign around to make a funny phrase. This pattern was bent in Captain Underpants and the Big, Bad Battle of the Bionic Booger Boy (part 2), which also introduces Melvin Sneedly, Mr. Krupp, Sulu, Carl, Trixie, and Frankenbooger, due to the chase. George and Harold still switch a sign around. This running gag was discontinued from the eighth book onwards due to the various situations and/or locations that are involved in the first chapter.
- Every book has two comics, one at the beginning (an intro) and one in the middle (for humor, and someone often complains about it afterward). The exceptions are book five where George and Harold create a third comic to save the world and book eight where the evil twins of George and Harold created a third comic.
- Every book has a chapter known as "The Incredibly Graphic Violence Chapter", which has an animation mode known as Flip-o-Rama or "The Cheesy Animation Technique". It is a method of animation by flipping one page with pictures on the page behind to create an illusion of repetitive action. There is a warning or notice right before the Flip-O-Rama occurs. The notices or warnings were replaced after the sixth book. In the seventh book, the Underpants Dance was explained and in the eighth book onwards, pictures are shown at the beginning of those chapters.
- There is occasional meta-humor; the most common joke being a character declaring something only happening in poorly written children's stories, usually right before the actual event happening.
- There is often a mention of things that happened in previous books (usually in the background). Starting with the sixth book, characters including the narrator will mention things that happened in the book prior to the book that is being read.
- In George and Harold's comics, the gym teacher and some mundane object are attacked somehow by the antagonist, and the principal expresses more concern towards the object rather the gym teacher. In the first book's comic, for example, the principal says "Oh, no! Not folding chairs!" after the children tell him what The Inedible Hunk has done to the school.
- All of George and Harold's comics have bad spelling and grammar. In the 8th book, it shows one of Evil George and Evil Harold's comic books about Captain Blunderpants. In it, it has correct spelling, grammar and better artwork. George and Harold humorously mention later that they thought some words were spelled wrong in it, and that the pictures looked horrible.
- In some books, there is a poster visible that says "GO Purple Dragon Sing Along Friends!" due to George and Harold switching a security camera tape with a Purple Dragon Sing Along Friends tape, so when Mr. Krupp brought it to the football team (The Knuckleheads), they liked it so much, they changed their names.
- There are many allusions in the series; for example, to Superman and Batman. Two of Harold's hamsters are named "Dr. Fine" and "Dr. Howard," the school the boys attend is called Jerome Horwitz Elementary, the initial team name is the Knuckleheads, and George and Harold get their hypnotic ring from the "Li'l Wise Guy Novelty Company," all references to The Three Stooges.
- The characters often refer to each other as "bub".
- In almost every book in the novel series there is a little boy along with his mother where the boy would say something about Captain Underpants, George, and Harold fighting someone and then the mother won't beleive him.
- George and Harold often note that the fate of the world is in their hands which is often said in a dire situation. The only times they have not said this were in the 6th and 7th books.