Billy Crystal voices Mike Wazowski, a green monster with a ball-shaped body, a single eyeball, two small horns, and skinny arms and legs.
John Goodman voices James P."Sulley" Sullivan, a giant furry blue friendly and sweet monster with horns and purple spots.
Steve Buscemi voices Randall 'Randy' Boggs, a purple, impatient multi-legged lizard-shaped monster with chameleon like abilities.
Mary Gibbs voices Boo, a 2-year-old human girl who was often scared by Randall.
James Coburn voices Henry J. Waternoose III, a five-eyed half-crab monster, who is the CEO of Monster Inc.
Jennifer Tilly voices Celia Mae, a one-eyed monster with snakes-like hair and tentacles for legs.
John Ratzenberger voices Yeti, a monster from Monstropolis who got banished to the Himalayas.