Aruego wrote 82 children's books. He lectured extensively at elementary schools, insisting on sketching a character into every book that he signed and teaching students to draw his alligators. Among his best known books were Whose Mouse Are You?, Leo the Late Bloomer and Gregory the Terrible Eater. He worked alongside his wife Ariane Dewey in creating books that were about animals. Their first book was published in 1969. In 2006, they reunited to produce The Last Laugh, a picture book that had a strong anti-bullying message.