A city of monsters with no humans called Monstropolis centers around the city's power company, Monsters, Inc. The lovable, confident, tough, furry blue behemoth-like giant monster named James P. Sullivan (better known as Sulley) and his wisecracking best friend, short, green cyclops monster Mike Wazowski, discover what happens when the real world interacts with theirs in the form of a 2-year-old baby girl dubbed "Boo," who accidentally sneaks into the monster world with Sulley one night. And now it's up to Sulley and Mike to send Boo back in her door before anybody finds out, especially two evil villains such as Sulley's main rival as a scarer, chameleon-like Randall (a monster that Boo is very afraid of), who possesses the ability to change the color of his skin, and Mike and Sulley's boss Mr. Waternoose, the chairman and chief executive officer of Monsters, Inc.
| John Goodman | James P. "Sulley" Sullivan (voice) |
| Billy Crystal | Mike Wazowski (voice) |
| Mary Gibbs | Boo (voice) |
| Steve Buscemi | Randall Boggs (voice) |
| James Coburn | Henry J. Waternoose (voice) |
| Jennifer Tilly | Celia (voice) |
| Bob Peterson | Roz (voice) |
| John Ratzenberger | The Abominable Snowman (voice) |
| Frank Oz | Fungus (voice) |
| Daniel Gerson | Needleman / Smitty (voice) |
| Steve Susskind | Floor Manager (voice) |
| Bonnie Hunt | Flint (voice) |
| Jeff Pidgeon | Bile (voice) |
| Samuel Lord Black | George Sanderson (voice) (as Sam Black) |
| Jack Angel | (voice) |
| Bob Bergen | (voice) |
| Rodger Bumpass | (voice) |
| Gino Conforti | (voice) |
| Jennifer Darling | (voice) |
| Patti Deutsch | (voice) |
| Pete Docter | (voice) |
| Bobby Edner | (voice) |
| Ashley Edner | (voice) |
| Paul Eiding | (voice) |
| Katie Scarlett | (voice) (as Katie Evans) |
| Bill Farmer | (voice) |
| Keegan Farrell | (voice) |
| Pat Fraley | (voice) |
| Teresa Ganzel | (voice) (as Terese Ganzel) |
| Taylor Gifaldi | (voice) |
| Marc John Jefferies | (voice) |
| Joe Lala | (voice) |
| Noah Luke | (voice) |
| Sherry Lynn | (voice) |
| Danny Mann | (voice) |
| Mona Marshall | (voice) |
| Mickie McGowan | (voice) |
| Laraine Newman | (voice) |
| Kay Panabaker | (voice) |
| Bret 'Brook' Parker | Trailer Mom (voice) (as Bret Parker) |
| Phil Proctor | Charlie (voice) |
| Josh Qualtieri | (voice) |
| Guido Quaroni | (voice) |
| Jan Rabson | (voice) |
| Lisa Raggio | (voice) |
| Joe Ranft | (voice) |
| Sophia Ranft | (voice) |
| Katherine Ringgold | (voice) |
| Bob Scott | (voice) |
| David Silverman | (voice) |
| Jim Thornton | (voice) |
| Lee Unkrich | (voice) |
| Randy Newman | Composer |
| Robert Grahamjones | Editor |
| Jim Stewart | Editor |
| Kent Brown | sound editor: temp effects |
| Lindakay Brown | foley editor |
| Jeanette Browning | dialogue recordist |
| Vince Caro | additional dialogue recordist |
| Derek Casari | adr engineer |
| Tony Eckert | foley mixer |
| Teresa Eckton | Sound Effects Editor |
| Sean England | machine room operator |
| Gert Janssen | dubbing editor |
| Doc Kane | original dialogue mixer |
| Noah Katz | sound editorial technical support |
| Stephen Kearney | Sound Effects Editor |
| Brian Magerkurth | sound re-recordist |
| John McGleenan | additional dialogue recordist |
| Frank 'Pepe' Merel | foley mixer |
| Shannon Mills | Sound Effects Editor |
| Tom Myers | Sound Designer |
| Morten Folmer Nielsen | adr editor / adr recorder |
| Jonathan Null | adr editor |
| Juan Peralta | digital sound mix technician |
| Mark Purcell | machine room operator |
| Gary Rydstrom | re-recording mixer / sound designer |
| Dee Selby | assistant sound designer |
| Michael Silvers | Supervising Sound Editor |
| Steve Slanec | supervising assistant sound editor |
| Gary Summers | re-recording mixer |
| Dennie Thorpe | foley artist |
| Jana Vance | foley artist |
| Karen G. Wilson | foley editor (as Karen Wilson) |
