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On The Simpsons, Castellaneta provides the voices for Homer Simpson, Abraham "Grampa" Simpson, Barney Gumble, Krusty the Clown, Groundskeeper Willie, Mayor Joe Quimby, Hans Moleman, Sideshow Mel, Itchy, Kodos, Arnie Pie, Scott Christian and other characters. For his work on the series, he won three Emmy Awards for Outstanding Voice-Over Performance. He has also written a few episodes with his wife and appeared as himself on the Simpsons episodes "Homer Simpson, This Is Your Wife", and "He Loves to Fly and He D'ohs".

He also played Grandpa and the mentally unstable ice cream truck driver known as "The Jolly-Olly Man" (among others) on Nickelodeon's Hey Arnold!, Doctor Emmett L. Brown in the animated series Back to the Future, the Things in The Cat in the Hat and Robot Devil in Futurama. In 1999 he played the comedic Nordom the anti-Modron in the computer game Planescape: Torment. Castellaneta was also the voice of Megavolt on Darkwing Duck, Genie in the Aladdin sequel Return of Jafar and on the Aladdin TV series and Dr. Zibaldo in the Talespin episode "The Incredible Shrinking Molly".

Castellaneta voiced the lead character in the animated series Earthworm Jim and narrated the movie Super Mario Bros. In 2005, he appeared on Arrested Development as a deadpan incompetent doctor who misdiagnosed Jason Bateman's character (and left tools inside him after surgery, causing him to exclaim "D'oh!") and in the movie The War of the Roses he seeks a divorce at the beginning of the movie. He also appeared in episodes of Frasier, Friends, Scrubs, Married... with Children (in the episode "The Dance Show", Castellaneta played the gay partner of a handsome man Peg dances with every night and he ends up cooking for Al), Yes, Dear, Reba, Everybody Loves Raymond, ALF, That '70s Show,and Stargate SG-1. (wikipedia.org)