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Rosemary Clooney George Clooney welcomed his aunt, famous '50s singer Rosemary Clooney, to the show in 1994. The White Christmas actress landed a reoccurring role as Mary Cavanaugh or Madame X, an elderly woman suffering from Alzheimer's. NBC - Getty Images

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Julianna Margulies The Good Wife actress's first big TV role was on ER as nurse Carol Hathaway. She was on the show until its sixth seasonalthough that wasn't what the producers originally planned. Margulies's character was supposed to die in the pilot episode, but the actress tested so well with audiences that her character was written into the series. NBC - Getty Images

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Busy Philipps From 2006 until 2007, Busy Philipps joined the medical staff at County General as Dr. Hope Bobeck. The actress had already gained notoriety on shows like Freaks and Geeks and Dawson's Creek. NBC - Getty Images

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Aaron Paul Before he became famous on Breaking Bad, Aaron Paul appeared on season 9 of ER, playing a teenager who was responsible for looking after his ill mother and younger siblings. NBC Universal

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Gloria Reuben In recent years, Gloria Reuben has appeared in shows like Mr. Robot and The Blacklist, as well as the Academy Award winning film Lincoln. But she's still best known for her role as Dr. Jeanie Boulet, for which Reuben received two Emmy nominations. NBC - Getty Images

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Zac Efron A few years before Zac Efron became Disney Channel's leading man in High School Musical, the young actor appeared in ER as Bobby Neville, a young boy who arrives at the hospital suffering from a gunshot wound. NBC Universal

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Erik Palladino Erik Palladino joined the ER cast in 1999. At the time, the role was a huge break for the emerging actor and kickstarted his career in both TV and film. Since leaving the show in 2001, Palladino has landed roles in NCIS: Los Angeles, The Marvelous Mrs. Maisel, and Suits. NBC - Getty Images

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John Stamos During a bit of a career slump in 2005, John Stamos joined the cast of ER to play Dr. Tony Gates. The former Full House star was a regular character on the show until leaving in 2009. NBC - Getty Images

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Shiri Appleby Shiri Appleby landed a reoccurring guest role as Dr. Daria Wade in 2008but that wasn't the first time the actress paid a visit to County General. In 1994, a young Appleby (pictured) played the patient Ms. Murphy in the episode "24 Hours." NBC Universal

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Thandie Newton Thandie Newton's reoccurring role on ER marked her first appearance on a U.S. series . The Westworld actress played Makemba Likasu, an administrator in the Democratic Republic of Congo's health ministry and love interest for Dr. John Carter (Noah Wyle). NBC - Getty Images

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Michael Michele Michael Michele appeared on ER from 1999 until 2001 as Dr. Cleo Finch. The actress, who earned her breakout role in Eddie Murphy's Harlem Nights, went on to star in How to Lose a Guy in 10 Days and Dark Blue, and as of late is a regular cast member on the Dynasty reboot. NBC - Getty Images

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William H. Macy William H. Macy snagged a reoccurring guest role on the medical drama as Dr. David Morgenstern, the chief of surgery and the head of the ER at County General Hospital. However, the actor's character was written out of the script in 1998. "My exit from the show was really weird. I was working enough in films that it was starting to become fractious," Macy told Archive of American Television in 2016 . "At any rate, I threw my hat in the ring to be a regular, I said 'let's set the number, or maybe I should leave.' And I never really heard back from anyone, but then one day in one of the scripts, I announced I was leaving." NBC - Getty Images

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Maura Tierney Maura Tierney made a name for herself with the comedy TV show, NewsRadio, which she was on from 1995 to 1999. Soon after leaving, she joined the cast of ER as Dr. Abby Lockhart. Tierney stayed at County General until season 15 and was most recently on The Affair. NBC - Getty Images

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Quentin Tarantino Who is better acquainted with the blood and gore of a medical drama than Quentin Tarantino? The writer-director was a guest director on the episode "Motherhood" in 1995. NBC - Getty Images

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Lucy Liu Before Charlie's Angels, the aspiring actress appeared on ER for a three episode arc as a hospital patient named Mei-Sun Leow. NBC Universal

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Linda Cardellini Linda Cardellini first appeared on ER as Samantha Taggart in season 10 and joined the cast shortly after as a nurse. She remained on the show until the series finale in 2009 and has gone on to have a successful career in TV and film, appearing in the 2018 Oscar-nominated movie Green Book. NBC - Getty Images

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Stanley Tucci Stanley Tucci appeared in season 14 of the series as Dr. Kevin Moretti, the Chief of Emergency Medicine. The actor arrived at County General in 2007 for a multi-episode arc. NBC - Getty Images

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Kirsten Dunst Kirsten Dunst had already risen to fame as a child star in Little Women and Interview with a Vampire, but in season 5 the actress appeared on the show for multiple episodes as a homeless teenager who befriends Dr. Ross (George Clooney). NBC Universal

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Shane West The A Walk to Remember actor joined the staff of County General Hospital in 2004 as Ray Barnett. Since then, he has found success in TV series like Nikita and Gotham. NBC - Getty Images

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Shia LaBeouf Before Shia LaBeouf earned his breakout role as a Disney Channel star on Even Stevens, the young actor appeared in season 6 of ER as a child suffering from muscular dystrophy. NBC Universal

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Alan Alda Alan Alda's (center) stint on ER in the show's fifth season may have reminded him of his time as a war doctor in M.A.S.H., unfortunately the actor's return to the silver screen was only a five episode arc. Getty Images - Getty Images

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Eric Stonestreet Before Modern Family, Eric Stonestreet guest starred on season 7 of the show as a Star Trek fan gone too far when he wound up in the emergency room for slicing his ears like Spock. NBC Universal

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Scottie Pippen Chicago Bulls player Scottie Pippen guest starred on the show as himself in 1996. The episode "Baby Shower" was a source of hometown pride, as the fictional hospital is based in Chicago. NBC - Getty Images

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Maria Bello Maria Bello left her position at County General Hospital in 1998 to pursue a career in film. The actress, who played the ER's pediatrician, Dr. Del Amico, found success in movies such as Coyote Ugly, Grown Ups, and The Mummy: Tomb of the Dragon Emperor. NBC - Getty Images

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Gabrielle Union Gabriella Union guest starred on ER in the season 6 episode "Family Matters," playing a high school student who was in a car accident. NBC Universal

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Ewan McGregor Before his role as Obi-Wan Kenobi was written in the stars, Ewan McGregor guest starred in a particularly tense episode in season 3 of the medical drama. McGregor played Duncan, a gunman in a convenience store robbery who forces Dr. Carol Hathaway (Julianna Margulies) to care for one of the victims. NBC - Getty Images

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Eva Mendes Before her breakout roles in films like Training Day and Hitch, Eva Mendes guest starred in season 3 of ER, playing a babysitter who brings the little girl she's caring for into County General with an E. coli infection. NBC Universal

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C.C.H. Pounder C.C.H. Pounder has appeared on shows like The X-Files, The West Wing, Law and Order: Special Victims Unit, and many more. But she's best known for Dr. Angela Hicks from ER. The actress left the show in season 4 to pursue opportunities in film and in 2009 landed a role in the Academy Award-winning film Avatar. NBC - Getty Images

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Sally Field Sally Field played the reoccurring role of Abby Lockhart's (Maura Tierney) mother, Maggie Wyczenski. Although her character's relationship with the rest of the characters was complicated, Field continued to make guest appearances from 2000 until 2006. NBC - Getty Images

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Rooney Mara Rooney Mara looks very different during her small guest spot on ER in 2009 from the character she played in her breakout film, The Girl With the Dragon Tattoo. In the episode, Mara plays a young mother who is deciding whether or not to give up her baby. NBC Universal

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Angela Bassett Joining the series in its final season, Angela Bassett entered County General as Cate Banfield, an emergency room attending physician. NBC - Getty Images

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Susan Sarandon In the final season, not only did former cast members George Clooney and Julianna Margulies return to the show for a number of episodes, but Susan Sarandon made a guest appearance as a grandmother who has to decide whether or not to pull the plug on her grandson and donate his organs. NBC Universal

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Chris Pine Before his days as a leading man, Chris Pine made an appearance on the show in season 9, as a drunk man who is sent to the emergency room. NBC Universal

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Ariel Winter Ariel Winter joined the cast of ER for a multi-episode story arc. The Modern Family star played a young girl whose mom was undergoing emergency surgery after the two were in a car accident. NBC Universal

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Forest Whitaker In season 13, the ward welcomed Oscar winner Forest Whitaker for a six episode guest appearance. Whitaker played former patient, Curtis Ames, who intends to sue the hospital staff for malpractice. NBC - Getty Images

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Kyle Richards Although Kyle Richards is best know for her reality TV career these daysshe's been on The Real Housewives of Beverly Hills since season 1she grew up on sets as a child actor and has continued to take on minor roles in adulthood. Richards landed a reoccurring role on ER as Nurse Dori and appeared on more than 20 episodes between 1998 and 2006. NBC Universal

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Sara Gilbert The former Rosanne star joined the ranks at County General, where she played the sarcastic medical student Jane Figler from 2004 until 2007. NBC Universal

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Khandi Alexander Before joining the cast in 1995 as Jackie Robbins, Khandi Alexander was on NewsRadio and in the film There's Something About Mary. After leaving the show in 2001, Alexander landed a reoccurring spot on Scandal. NBC - Getty Images

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Elizabeth Mitchell In 2006, Elizabeth Mitchell landed her most well-known role on the series Lost. But before that, ER fans knew her from her 14-episode reoccurring role as Dr. Kim Legaspi. NBC Universal