* CompleteMonster: See [[Monster/TheHardyBoys here]].
* CrossoverShip: With Nancy Drew, with whom the Hardys have many different crossover works. This is particularly true of the first Supermysteries series, which crosses the ''Casefiles'' and ''Nancy Drew Files'' together; in canon, Frank's in love with Callie and Nancy's in love with Ned, and while Frank and Nancy do share a mutual attraction, they're too loyal to their respective partners to pursue it. In fan works, there are plenty of fanfiction stories that ditch Callie and Ned to pair Frank and Nancy together.
* EnsembleDarkhorse:
** Vanessa Bender, Joe's second long-standing girlfriend (i.e. the first since Iola who isn't just a GirlOfTheWeek) in the ''Casefiles'', who, despite not having appeared in the books since that series's cancellation over twenty years ago, still makes regular appearances in fanfic.
** Chet Morton. ''Dear God'', '''Chet'''. He has appeared in the most books out of any non-Hardy, and was so popular that in 1965, they were considering giving him his ''own book series'', which [[DevelopmentHell never got past the planning stage.]]
* FairForItsDay:
** Although Tony and Phil were always referred to as the "Italian friend" and "Jewish friend" in the early books and there was some mild stereotyping, they were also two of the first positive portrayals of non-WhiteAngloSaxonProtestant characters in the early twentieth century, and were otherwise treated as no different than any of the other teenagers in the Hardys' circle of friends.
** While the treatment of African-Americans, Native Americans, and Asians were pretty racist, the treatment of Hispanics, including Mexicans, were respectful during the early years of the books.
* FriendlyFandoms: With ''Literature/NancyDrew''. The fact that they have crossed over ''numerous'' times has only given more fuel to this.
* HarsherInHindsight: Considering the sheer number of stories that have been written, it's almost inevitable that this will eventually come up. One good example is "Stress Point" from the ''Casefiles''. The details of the book are different (A reporter is murdered after he finds out that a construction company in bed with the mob cut corners on a new skyscraper, which makes it dangerous in an upcoming storm), but it doesn't change the fact that the story ends with a building beginning to collapse in the middle of New York City.
* MyRealDaddy: While Edward Stratemeyer is the actual creator of the Hardys, all he did was create a rough idea of the character and general outlines for stories for his ghostwriters to follow. It was Leslie [=McFarlane,=] the first ghostwriter, who (after originally working on another series called ''Dave Fearless'' and finding himself disappointed at how slapdash and generic it was treated) added a lot more beyond his outline and really brought them to life. Ironic since he later grew to absolutely despise them.
* NightmareFuel:
** [[https://i.gr-assets.com/images/S/compressed.photo.goodreads.com/books/1328718453i/76984._UY475_SS475_.jpg This 1962 cover]] for "While The Clock Ticked".
** Cliffhanger already has a tense premise where a mountaineering expedition is tormented by an abominable snowman. Then there's the fact that, at one point Joe gets separated from his companions and lands in the "monster's" hideout, and the creature itself stops by soon enough (although at this point Joe also learns that the monster was just a hoax). The real clincher is in the finale, where an accident with the ropes had Frank dangling on a steep cliff wall, and he sees a ''frozen corpse'', embedded in the ice. Frank only had a brief glimpse, but he monologues that the image will stay with him for life.
* ValuesDissonance: The original editions published before 1959, which featured plenty of stereotypes, particularly of different nationalities, and racist attitudes, especially towards African-Americans, Native Americans, and Asians. The post-1959 revised editions took them out. Unfortunately, instead of just removing the stereotypes, the revised editions feature few, if any, non-white characters altogether, which was considered the more politically correct option at the time.
* VindicatedByReruns: When the books first came out, they were considered worthless garbage that ruined young readers' taste for "proper literature." However, by the time of UsefulNotes/TheSeventies and the predominance of television, the success of the TV adaptation, ''Series/TheHardyBoysNancyDrewMysteries'' lead to parents cheering that their kids might like to read the novels to enjoy some kind of literature.
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