The Hardy Boys, Frank and Joe Hardy, are fictional teenage brothers and amateur detectives who appear in various mystery series for children and teens. The characters were created by Edward Stratemeyer, the founder of the Stratemeyer Syndicate, a book-packaging firm, and the books have been written by many different ghostwriters over the years. The books are published under the collective pseudonym Franklin W. Dixon.

The Hardy Boys have evolved in various ways since their first appearance in 1927. Beginning in 1959, the books were extensively revised, largely to eliminate racist stereotypes. The books were also written in a simpler style in an attempt to compete with television. Some critics argue that in the process the Hardy Boys changed, becoming more respectful of the law and simultaneously more affluent, "agents of the adult ruling class" rather than characters who aided the poor. A new Hardy Boys series, the Hardy Boys Casefiles, was created in the 1980s, and featured murders, violence, and international espionage. The original Hardy Boys Mystery Stories series ended in 2005. A new series, Undercover Brothers, was launched the same year, featuring updated versions of the characters who narrate their adventures in the first person.