List of guest stars
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This is a list of guest stars who appeared on The Simpsons. Many episodes of iconic animated series The Simpsons feature celebrity guests contributing their voices to the show, whether as themselves or as fictional characters. This list does not include those celebrities whose voices have merely been impersonated. For the 20th anniversairy they made a special episode based on all the celebrities who have guest starred in The Simpsons.
There are several non-regular voices who make appearances but are credited as "also starring" and not guest voices. These voices are not mentioned here. As well, Marcia Wallace has made many de facto appearances and has always been credited as a guest star. Only the episodes in which her character Edna Krabappel plays a significant role are mentioned here. This is partially because the list becomes a lot more cluttered because she has guest starred in close to a hundred episodes, so although she is only credited a few times here, she has "guest starred" in many episodes.
NOTE: Michael Jackson was credited as John Jay Smith for this appearance. Although he provided his own voice for the dialogue, Kipp Lennon, a Jackson impersonator, did all the singing.
NOTE: Glenn Close makes an appearance in this episode in a deleted scene that was originally intended for "Mother Simpson". So, although it was not recorded specifically for this episode, her appearance is noted here. The same is true for Phil Hartman as Lionel Hutz. Buzz Aldrin's appearance is from "Deep Space Homer" and Liz Georges is from short "Good Night".
NOTE: In this episode, the individual members are not credited and only "U2" is listed. Larry Mullen does not have any lines in the episode and thus is not credited here.
NOTE: The episode "A Tale of Two Springfields" featured John Entwistle and Roger Daltrey of The Who, but for unknown reasons, Pete Townshend did not lend his voice. Pete did appear in the episode but was voiced by his brother Paul Townshend. In the actual episode, all three were simply credited as "The Who". In a normal episode the group name plus all the voices are listed, but it was not done in this case likely to avoid confusion. 
NOTE: In the end credits, Jay Mohr is actually credited with the voice of Christopher Walken, the credit says "Jay Mohr as Christopher Walken". This is the first time this has ever been done.
NOTE: All except NRBQ's and Deb Lucasta's appearance were from previous episodes. U2 was not credited as individual members, with Larry Mullen not having any lines. Taken from "When You Dish Upon A Star", "New Kids on the Blecch", "Trash of the Titans", "They Saved Lisa's Brain", "Hello Gutter, Hello Fadder", "I'm with Cupid", "Treehouse of Horror X", "Bart Star" and "Lisa's First Word", respectively.
NOTE: Taken from "Dancin' Homer".
 Season 23
 Season 24
 Guest stars that have appeared multiple times
NOTE: This list does NOT include unaired episodes, re-used footage, The Simpsons Movie or The Simpsons Game.