Happy Tree Friends (HTF) is an adult animated web series created by Aubrey Ankrum, Rhode Montijo, and Kenn Navarro, and developed by Montijo, Navarro, and Warren Graff for Mondo Media. Disguised as a children's cartoon, the series follows the misadventures of cute anthropomorphic forest animals, who live initially peaceful lives until they are killed or injured in sudden, usually accidental, graphically violent incidents. Debuting in December 2000, Happy Tree Friends has achieved a cult following on Mondo's website and YouTube channel and expanded into a multimedia franchise, which includes the television series of the same name.

While working with Mondo Media, Rhode Montijo drew a character on a piece of scrap paper who would later become Shifty. He then drew a yellow rabbit that bore some resemblance to Cuddles, writing "Resistance is futile" underneath it on a spreadsheet poster. Rhode hung the drawing up in his workstation so other people could see his idea, and eventually, the idea was pitched to and accepted by the Mondo Media executives. In 1999, Mondo gave Aubrey Ankrum, Rhode Montijo, and Kenn Navarro a chance to do a short for them. They came up with a short named "Banjo Frenzy", which featured a dinosaur (an earlier version of Lumpy) killing three woodland animals, a squirrel, a rabbit, and a beaver (earlier versions of Giggles, Cuddles, and Toothy) with a banjo. From there, Mondo gave them their own Internet series, which they named Happy Tree Friends.

Although the first episodes of Happy Tree Friends were produced in 1999, it debuted online sometime in 2000 and became an unexpected success, getting over 15 million hits each month along with screenings at film festivals.

Mondo Media CEO John Evershed attributes the success of the series to animator Kenn Navarro:

"He had a clear vision for that show and he's just a brilliant animator. He has created something that is pretty universal. I envision kids watching Happy Tree Friends 20 or 30 years from now the same way that they watch Tom and Jerry now. So really it's Kenn Navarro."

In 2014, after the episode "Dream Job" was released, Mondo Media announced plans to produce a feature film based on the series, but in 2016, Kenn Navarro tweeted that he was unaware of work being done on a film, but that his team was "in talk to do more shorts". Later, when a fan asked Kenn Navarro about the film, he replied: "a treatment that [I] and the writers did was all the work (that [I] know of) for the movie."

Throughout December 2016, Mondo Media released five all-new episodes for purchase online. Bundled as a set named "Happy Tree Friends: Still Alive", the episodes came with some additional bonus material such as background designs, animated storyboards, the animation process, and a writer's session video. Upon purchase, the buyer was allowed to download the DRM-free video files to their computer. In January 2017, Kenn Navarro tweeted "As I understand, sales were OK but fell below what was expected." On December 22, 2016, the Happy Tree Friends Internet series from 1999 ended its run. Meaning a Season 6 will not happen, however they still release content to this day.

It was announced on August 17, 2023, that a new Happy Tree Friends episode titled "Too Much Scream Time" would be released on September 27, 2023 to tie into the downloadable content for the shoot 'em up video game The Crackpet Show (titled The Crackpet Show: Happy Tree Friends Edition), which was released on the same day. On October 18, 2024, a new episode titled "Happy Train Friends" was released as a crossover with Dumb Ways to Die.