The Grim Adventures of Billy & Mandy is an American animated television series created by Maxwell Atoms for Cartoon Network. It follows Billy, a dimwitted, happy-go-lucky boy, and Mandy, a cynical, remorseless girl, who, after winning a limbo game to save Billy's pet hamster, gains Grim, the mighty Grim Reaper, as their "best friend forever". Grim, who is reluctant to serve the two children, has access to supernatural items, spells, and other abilities that often lead Billy and Mandy to interact with otherworldly environments, characters, or situations.

Billy & Mandy began as a segment on Grim & Evil, from which it was a spin-off, along with sister series Evil Con Carne, on August 24, 2001. Although the 2003–2004 episodes were produced for Grim & Evil, the series aired (as a separate series) from June 13, 2003, to October 12, 2008, on Cartoon Network. In 2007, the series produced two made-for-TV movies, respectively titled Billy & Mandy Save Christmas and Billy & Mandy's Big Boogey Adventure. Two crossover specials with fellow Cartoon Network series Codename: Kids Next Door, entitled "The Grim Adventures of the KND", aired on November 11, 2007, and its sister series Evil Con Carne, entitled "Company Halt", aired on March 16, 2007. The series finale, a made-for-TV movie titled Underfist: Halloween Bash (intended to serve as a pilot for a new spin-off series), aired on October 12, 2008.

During its run, the series won two Emmy Awards and one Annie Award, with nominations for one Daytime Emmy Award, three Golden Reel Awards, and two other Annie Awards. Billy & Mandy has also been made into a video game as well as various licensed merchandise.