The Grim Adventures of Billy & Mandy is an American animated television series created by Maxwell Atoms for Cartoon Network and distributed by Warner Bros. Domestic Television Distribution. It follows Billy, an extremely dimwitted, happy-go-lucky boy, and Mandy, a cynical, remorseless girl, who, after winning a limbo game to save Billy's pet hamster, gain 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 series of segments 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 show ran as a separate series from June 13, 2003, to November 9, 2007, on Cartoon Network. Two made-for-TV movies were also produced, Billy & Mandy's Big Boogey Adventure, which aired on March 30, 2007, and Billy & Mandy: Wrath of the Spider Queen, which aired on July 6, 2007. A crossover special with fellow Cartoon Network series, Codename: Kids Next Door, entitled "The Grim Adventures of the KND", aired on November 11, 2007. One more made-for-TV movie, 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.