Adam Lambert(01 of39)
Open Image ModalGlambert on "Glee." This hadn't happened earlier? (credit:Getty)
Barry Bostwick(02 of39)
Open Image ModalThe original Brad Majors made a cameo as Tim Stanwick in Season 1's "The Rocky Horror Glee Show" episode. (credit:Getty)
Brian Stokes Mitchell(03 of39)
Open Image ModalThe Broadway star played Leroy, one of Rachel Berry's two gay dads. (credit:Getty)
Britney Spears(04 of39)
Open Image ModalThe show covered several of the pop star's songs and dedicated two episodes to her. (credit:Getty)
Carol Burnett(05 of39)
Open Image ModalThe famed comedienne played Sue Sylvester's mother, Doris, in "Furt" during Season 1. (credit:Getty)
Charice(06 of39)
Open Image ModalThe pint-size powerhouse sang her heart out as Sunshine Corazon. (credit:AP)
Cheyenne Jackson(07 of39)
Open Image ModalThe singer, Broadway actor and international ambassador for amfAR also added "Glee" to his repertoire: he guested several episodes in Season Three as Dustin Goolsby. (credit:Getty)
Eve(08 of39)
Open Image ModalHip-hop's royalty graced "Glee" with her presence during Season 2 as Grace Hitchens. (credit:Getty)
Gloria Estefan(09 of39)
Open Image ModalThe role of Santana's mom, Maribel Lopez, went to the legendary singer. (credit:AP)
Gwyneth Paltrow(10 of39)
Open Image ModalHolly Holliday seduced Will Schuester in Season 3. (credit:Getty)
Helen Mirren(11 of39)
Open Image ModalBecky's inner monologue is voiced by the Dame. (credit:AP)
Idina Menzel(12 of39)
Open Image ModalShelby Corcoran, Rachel Berry's biological mother, sang with her daughter throughout the show. (credit:Getty)
James Lipton(13 of39)
Open Image ModalHe was credited as a special appearance on Season 1's "Goodbye." (credit:Getty)
Jeff Goldblum(14 of39)
Open Image ModalGoldblum guested as Rachel's second dad, Hiram Berry. (credit:Getty)
Jonathan Groff(15 of39)
Open Image ModalThe "Spring Awakening" hunk reunited with costar Lea Michele on the show. He fought for Berry's love as Jesse St. James. (credit:Getty)
John Lloyd Young(16 of39)
Open Image ModalThe "Jersey Boys" star got the Acafellas' kabosh. (credit:Getty)
John Michael Higgins(17 of39)
Open Image ModalThe hilarious actor played Russell in "Dream On." (credit:Getty)
John Stamos(18 of39)
Open Image ModalStamos shook things up between Emma and Will as sexy dentist Carl Howell on Season 4. (credit:Getty)
Josh Groban(19 of39)
Open Image ModalHis presence caused Sandy Ryerson to spout the unforgettable line, "Who is Josh Groban!? Kill yourself." (credit:Getty)
Kathy Griffin(20 of39)
Open Image ModalShe was Tammy Jean Albertson on Season 1's "Original Song." (credit:AP)
Kate Hudson(21 of39)
Open Image ModalHudson made life hell for Rachel Berry as her NYADA instructor Cassandra July. (credit:AP)
Katey Sagal(22 of39)
Open Image ModalShe transformed into Artie's mom, Nancy Abrams. (credit:Getty)
Katie Couric(23 of39)
Open Image ModalThe journalist extraordinaire chatted with Sue Slyvester on Season 1. (credit:Getty)
Kristin Chenoweth(24 of39)
Open Image ModalChenoweth lent her booming talent to the show as April Rhodes. (credit:Getty)
Lindsay Lohan(25 of39)
Open Image ModalRemember her first season role as a Nationals judge? (credit:AP)
Loretta Devine(26 of39)
Open Image ModalDuring Season 1, the award-winning actress was Sr. Mary Constance on the "Original Song" episode. (credit:Getty)
Matt Bomer(27 of39)
Open Image ModalBomer played big brother to Darren Criss as Cooper Anderson. (credit:Getty)
Meat Loaf (28 of39)
Open Image ModalThe rocker played Barry Jeffries on "The Rocky Horror Glee Show." (credit:Getty)
Molly Shannon(29 of39)
Open Image ModalShe had a hilarious run as teacher Brenda Castle. (credit:AP)
Neil Patrick Harris(30 of39)
Open Image ModalSchuester dueled to the singing death with his nemesis Bryan Ryan during "Dream On." (credit:Getty)
Nene Leakes(31 of39)
Open Image ModalLeakes went for the jugular as Sue's archenemy Roz. (credit:Getty)
Olivia Newton-John(32 of39)
Open Image ModalThe Grease star acted as herself. (credit:Getty)
Patti LuPone(33 of39)
Open Image ModalLuPone was credited for a special appearance as herself in Season 1. (credit:Getty)
Perez Hilton(34 of39)
Open Image ModalAnother special appearance credit went to the celebrity blogger (Season One). (credit:Getty)
Rex Lee(35 of39)
Open Image ModalLloyd Lee? No, it's Martin Fong from "Nationals." (credit:Getty)
Ricky Martin(36 of39)
Open Image ModalHottest Spanish Teacher at McKinley High goes to David Martinez. (credit:Getty)
Sarah Jessica Parker(37 of39)
Open Image ModalThe actress went full-on fashion as Isabelle Wright in Season 3. (credit:AP)
Victor Garber(38 of39)
Open Image ModalWho else could pull off being Will Schuester's dad? (credit:Getty)
Whoopi Goldberg(39 of39)
Open Image ModalOne thing stood in Rachel Berry's quest for NYADA: Carmen Tibideaux. (credit:Getty)