Monsters, Inc. (also known as Monsters, Incorporated) is a 2001 American animated comedy film produced by Pixar Animation Studios for Walt Disney Pictures. Featuring the voices of John Goodman, Billy Crystal, Steve Buscemi, James Coburn, Mary Gibbs, and Jennifer Tilly, the film was directed by Pete Docter, co-directed by Lee Unkrich and David Silverman, and produced by Darla K. Anderson, from a screenplay by Andrew Stanton and Daniel Gerson. The film centers on two monsters, the hairy James P. "Sulley" Sullivan (Goodman) and his one-eyed partner and best friend Mike Wazowski (Crystal), who are employed at the titular energy-producing factory Monsters, Inc., which generates power by scaring human children. However, the monster world believes that the children are toxic, and when a little human girl, Boo (Gibbs), sneaks into the factory, she must be returned home before it is too late.

Docter began developing the film in 1996, and wrote the story with Jill Culton, Jeff Pidgeon and Ralph Eggleston, following the idea conceived in a lunch in 1994 attended during the near completion of Toy Story (1995). Stanton wrote the screenplay with Gerson. The characters went through many incarnations over the film's five-year production process. The technical team and animators found new ways to simulate fur and cloth realistically for the film. Randy Newman, who composed the music for Pixar's three prior films, returned to compose the score for its fourth.

Monsters, Inc. premiered at the El Capitan Theatre in Los Angeles on October 28, 2001, and was released in theaters in the United States on November 2. Upon its release, it received critical acclaim and was a commercial success, grossing over $528 million worldwide to become the third-highest-grossing film of 2001. The film won the Academy Award for Best Original Song for "If I Didn't Have You" and was nominated for the first Best Animated Feature, but lost to DreamWorks' Shrek, and was also nominated for Best Original Score and Best Sound Editing. Monsters, Inc. saw a 3D re-release in theaters on December 19, 2012. A prequel titled Monsters University, which was directed by Dan Scanlon, was released on June 21, 2013. A sequel series titled Monsters at Work premiered on Disney+ on July 7, 2021.