The Rebounding Ryan Murphy Star
After the WB canceled Ryan Murphy's first series, Popular, in 2001, his leading lady Leslie Bibb rebounded with an eight-episode stint on the NBC drama as med student Erin Harkins, who romanced Goran Visnjic's Luka before nearly dying in a car accident and leaving town. Similarly, after Murphy's Glee wrapped in 2015, his star Matthew Morrison chose the ABC soap as one of his first few high-profile guest-star gigs, appearing in four episodes as the abusive ex-husband of Camilla Luddington's Jo.
The Future Couple
In 1999, Holland Taylor appeared in one episode of the NBC series as County General patient Phyllis Farr. Meanwhile, her future girlfriend Sarah Paulson popped up in the ABC series in 2010 as a far more important character—a young Ellis Grey, mother to Ellen Pompeo's Meredith Grey. The two began dating in early 2015.
The Former Gilmore Girls Star
Two years after her last regular episode as Rory Gilmore, Alexis Bledel returned to TV as Dr. Julia Wise in the NBC hit's very last episode ever. Meanwhile, her TV grandpa Edward Hermann showed up on the ABC soap in 2007 as surprisingly mature intern Norman Shales. Only one would return to their Gilmore roles when Netflix revived the series for 2016's Gilmore Girls: A Year in the Life. Hermann had passed away on Dec. 31, 2014 at the age of 71.
The Future Friday Night Lights Star
Before he was Ornette Howard in the fourth season of NBC's beloved football drama, Cress Williams appeared in 18 episodes of ER as Officer Reggie Moore. And before he was Coach, Kyle Chandler played bomb squad leader Dylan Young in Grey's' season two post-Super Bowl episode. Only one of them literally blew up though.
The Future Scandal Star
Before he was Huck on Shonda Rhimes' twisty political thriller, Guillermo Diaz booked a 1995 episode of ER for what would be one of his earliest TV roles. And before they were the devious Cyrus Beene and Jake Ballard, respectively, Jeff Perry and Scott Foley popped up on Rhimes' first ABC series. Perry famously played Meredith's estranged father Thatcher, while Foley guested as Henry, the ill-fated husband of Kim Raver's Teddy Altman.
The Addams Family Values Star
Nothing creepy or kooky about this! Years after they appeared in the 1993 sequel as Wednesday Addams and her Camp Chippewa love interest Joel Glicker, Christina Ricci and David Krumholtz popped up in one of TV's long-running medical dramas a piece. Krumholtz arrived in season six of the NBC series as schizophrenic patient Paul Sobriki who, you might recall, was the one who broke our hearts when he stabbed Kellie Martin's poor Lucy Knight to death during a psychotic break. Ricci's arrival on Grey's in season two was not quite as heartbreaking—she played the paramedic with her hand dangerously trapped on an explosive in the post-Super Bowl episode—though it was just as intense.
The Keke Palmer
Keke Palmer is one of the few actors who have appeared on both shows. In one of her earliest acting gigs, she appeared in one of Noah Wyle's last episodes ever as a series regular after a decade of playing Dr. John Carter. Nearly a decade later, she would appear in a 2014 episode of Grey's as well.
The Future Frank Underwood Victim
Before either of them were—spoiler alert!—meeting grisly demises on Netflix's House of Cards, both Corey Stoll and Neve Campbell made medical show guest appearances. Stoll showed up in one 2005 episode of ER as Dr. Teddy Marsh, while Campbell appeared in two season nine episodes of Grey's as Liz Shepherd, one of four sisters belonging to Patrick Dempsey's late Derek.
The Future The Help Star
Before either Octavia Spencer or Jessica Chastain could bond as Minny Jackson and her employer Celia Foote in 2011's The Help, they both appeared on ER early in their careers. Meanwhile, their co-star Chris Lowell—who played the boyfriend of Emma Stone's Skeeter Phelan—appeared in two episodes of Grey's before moving on to spinoff Private Practice for another 54 more as Dell.
The SNL Star
A year before she joined the cast of Saturday Night Live in 2009, Nasim Pedrad recurred on what would become her home network's medical drama as Nurse Suri. Meanwhile, seven years after she was let go from the late-night comedy, Casey Wilson popped up as a patient pregnant with quadruplets on the ABC soap, returning to the network three years after it canceled her beloved Happy Endings.
The Broadway Legend
Who doesn't love a Tony-nominated star of stage? In 1999, Martha Plimpton began a four-episode stint on the NBC drama as pregnant out-of-work waitress Meg whom Julianna Marguiles' Nurse Carol Hathaway tries to help get on her feet. Meanwhile, the legendary Bernadette Peters arrived on Grey's in 2008 for a two-part season five episode.
The Future Breaking Bad Star
Before either would go on to have their lives ruined by Walter White as his wife Skyler and former student-turned-fellow meth cook Jesse Pinkman on the iconic AMC drama, Anna Gunn and Aaron Paul would pop up on NBC's medical drama in 1999 and 2003, respectively. Meanwhile, before Jesse Plemmons sent chills down our spines as he charming psychopath Todd Alquist in Breaking Bad's final season, he appeared in a 2006 episode of the ABC soap.
The Chris
Every show needs a visit from a famous Chris. ER got theirs in 2003 when Chris Pine appeared in an episode in what would be his first acting role. Grey's, on the other hand, saw their famous Chris arrive in the form of Chris O'Donnell, who recurred as Meredith's vet and short-lived love interest Finn Dandridge.
The Ally McBeal Lovers
Once upon a time, Lucy Liu and Greg Germann got frisky on camera as Ally McBeal's oddball Ling Woo and Richard Fish, respectively. They also each appeared on one of the shows in question, with Liu appearing in three 1995 episodes of ER, while Germann currently recurs as Dr. Tom Koracick on Grey's.
The Josh Radnor
Another of the few actors to appear on both shows, Josh Radnor played the gay lover of a Chicago alderman who contracted syphilis in ER's ninth season. Meanwhile, he popped up on Grey's in 2018 as one of Meredith's ill-fated season 15 dates.
The Jeffrey Dean Morgan
We all know Jeffrey Dean Morgan played the heartbreaking Denny Duquette in season two of Grey's—and again during season five's ill-advised ghost sex storyline—but did you know he appeared on ER first? In a 2001 episode, Morgan guest-starred as a firefighter.
The Future Avenger
Before they joined the Marvel Cinematic Universe as War Machine and and Black Panther, respectively, Don Cheadle and Chadwick Boseman both appeared on the NBC drama. Cheadle appeared in four episodes in 2002 as Dr. Paul Nathan, while Boseman appeared in one 2008 episode as a patient named Derek Taylor. Meanwhile, over on Grey's, a pre-Valkyrie Tessa Thompson appeared in two 2006 episodes as Camille Travis, niece of James Pickens, Jr.'s Richard Webber.
The Jessica Capshaw
While we all know that Jessica Capshaw starred as the beloved Dr. Arizona Robbins for 10 seasons on Grey's Anatomy, she also holds the distinction of being the only series regular on either show to have guest starred on the other. The actress appeared in one 1999 episode of ER as Sally McKenna.
The Oscar Winner
When ER needed someone to come in to play Maggie Wyczenski, the bipolar mother of Maura Tierney's Abby Lockhart, they turned to Oscar-winning legend Sally Field, who played the character in 12 episodes from 2000-2006. Similarly, when Grey's Anatomy needed someone to guest star in season 12 as the angry wife of a patient who shows up at the hospital wondering if her husband is dead yet, they looked no further than EGOT icon Rita Moreno. Sadly, Moreno's Gayle only appeared in the one incredible episode.
The Burgeoning Child Star
Audiences got their first taste of future child star Dakota Fanning when she appeared in a 2000 episode of ER, playing Delia Chadsey, a car crash victim suffering from leukemia. Grey's, meanwhile, gave us one of our earlier glimpses at the preternatural talents of Abigail Breslin when she appeared in a 2006 episode as Megan Clover, a young patient who can't feel pain.
The TV Legend
It was a return to the genre that made him a star when M*A*S*H icon Alan Alda joined ER for a five-episode stint in 1999 as Dr. Gabriel Lawrence, County General's new attending physician who immediately clashed with Anthony Edwards' Mark Greene and was ultimately diagnosed with Alzheimer's disease. Meanwhile, Happy Days fans were in for a treat when Mrs. C herself Marion Ross showed up in a 2010 Grey's episode as Betty Donahue, an elderly patient who reunites with her long-lost love after they're both randomly admitted to the E.R. at the same time.
The Future Man Men Shero
Before they were doing what they had to do to get by in a man's world on the iconic AMC series, Christina Hendricks and Elisabeth Moss each appeared on one of TV's longest-running medical dramas. Hendricks appeared in four episodes of ER in 2000, playing Joyce Westlake, Abby's new neighbor stuck in an abusive relationship with her boyfriend. Meanwhile, Moss appeared in one 2007 episode of Grey's as Nina Rogerson, the daughter of a patient who sadly died after a risky surgery.
The Burgeoning Teen Star
Before she was starring in hits like Bring It On and Dick, Kirsten Dunst had a recurring role on ER, playing Charlie Chemingo, a child prostitute who was cared for by George Clooney's Dr. Doug Ross. Dunst appeared in a total of six episodes between 1996-97. Similarly, before she visited the Upside Down as Stranger Things' breakout character Eleven, Millie Bobby Brown appeared in a 2015 episode of Grey's as Ruby, a young girl who needs the help of Kevin McKidd's Dr. Owen Hunt after an earthquake hits Seattle.
The Roswell Leading Lady
Two Roswells means two Lizs, and oddly enough, each of them have appeared on one of these shows. Shiri Appleby, the OG Liz who starred on the WB/UPN series from 1999-2002, actually appeared on ER twice in her career. Her first appearance, as a teen with ectopic pregnancy, took place in the 1994 series premiere, while her second, as intern Daria Wade, took place over the course of nine episodes in the show's final season. Meanwhile, Jeanine Mason, Liz in the CW's reboot Roswell, New Mexico, recurred as Dr. Sam Bello for 12 episodes of Grey's from 2017-18.