Ellen Pompeo, Kate Walsh and Chandra Wilson Pic credit: NBC

Over its seventeen seasons, Greys Anatomy has seen a score of actors pass through its walls. Some of them were unknowns when they joined the show while others were TV veterans.

Given their long histories, it should be no surprise that some Greys folks popped up in the other long-running TV franchise, Law & Order. Given that the shows been running since 1990 with several spinoffs, the odds of a few Greys actors dropping in on L&O is obvious.

It can be fun seeing some future Greys stars popping up in an L&O episode. There are also times when a former Greys star guested on L&O after leaving Seattle-Grace.

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Whats more notable is that some of Greys actors didnt just do one appearance but multiple ones as different characters. At least one Greys star can claim a trifecta of L&O appearances and a few others are close behind.

Here are 10 Greys Anatomy stars fans may have forgotten were on Law & Order.

Isaiah Washington can be a controversial name for Greys fans. While his role as Burke was a key early role for the series, Washington got in trouble for some backstage comments made about his castmates that led to his exit.

Washington already had a good TV career with a brief stop in the second season L&O episode Out of Control. The detectives have to investigate a brutal attack on a young woman with Washington among the suspects.

After leaving Greys, Washington guest-starred in the L.A. spinoff. As with many L&O cases, what looked like a simple murder ended up more complex as the victim had been working on a controversial voting measure.

Washington was a worker of the victim, which lead to him being arrested. His reverend father (Charles S. Dutton) defended him in court, but it wasnt long before the cops figured there was a bigger case.

The actor was good, making viewers wonder if he was guilty or not, which added to the complexity that made Burke so divisive.

This season, Greys fans had to say goodbye to Jesse Williams as Jackson Avery left the hospital to become an advocate.

As it happens, Williams very first acting credit came with Law & Order. It was a brief role in the opening scene of America Inc.

Williams was among the friends watching a basketball game on TV when a bullet ripped through the set without warning.

It was from the shooting of a military contractor next door that sent the cops on a complex case. It got more complicated when McCoy faced the military and the state department on a possible terrorist attack.

It was just a few moments, but it did get Williams foot in the door on TV for a fun future famous face cameo.

The latest season of Greys had a brief return of Sarah Drews loveable and bright character April. She sure wasnt innocent in the SVU episode Responsible.

When a teenager died after too much drinking, the team found a pack of party-loving classmates. Drew played Becca, the more innocent of the bunch.

She was as shocked as the audience coming home and finding her boyfriend in bed with her own mother who was the one supplying the teens with alcohol.

As the case continued, it turned out Becca was an even bigger lush than any of her classmates which lead to tragedy.

Its a striking image of a more innocent-looking Drew who turned out to have a lot of darkness in her to fit this difficult case.

Greys fans have come to know Greg Germann as the arrogant Tom Koracick. But SVU fans got to love to hate him first in a few episodes as Derek Strauss.

He first arrived in Undercover Blue as a special prosecutor who focused on police-related cases. He tried Brian Cassidy for assault, pushing him as a true scumbag with the team barely able to prove Cassidys innocence.

Then in Amaros One-Eighty, Strauss tried to indict Amaro for a hate crime after he shot a suspect. Straus topped himself in Post-Mortem Blues by trying to indict Benson for murdering a suspect.

Season 18s Genes revealed Strauss had switched sides to become a defense attorney. Its thus possible that Germann returns to plague the SVU team once more.

Law & Order loves to cast actors from New Yorks vibrant Broadway community. That afforded Sara Ramirez a chance to appear on SVU.

At the time, the actress was a few years off from her Tony-winning performance in Spamalot. She had a brief role at the start of the SVU episode Chameleon as a worker at a party the cops busted up.

She claimed one cop felt her up, which got SVU involved. They figured the girl was just making it up to avoid jail time. But she got involved in areal case when another womans body was found on the scene.

It was a brief role that got Ramirez attention before landing the more familiar part of Callie on Greys, which made her a huge fan favorite.

Teddy Altman has been a great character on Greys from wrestling with feelings between Owen and David to personal drama. Kim Raver is a TV veteran of several series, so its logical a L&O appearance would be among them.

In Homesick, when an infant died, the police suspected his British nanny abused him. Raver played the boys mother, naturally grief-stricken, and wanted someone to pay for it.

As the case continued, it became clear the mother was more interested in her career than her child and the defense even suggested she was to blame.

Raver is gorgeous in the role but also had a cold side that leaves the audience guessing with the cops. Even if shes not the perp, its clear by the end she played a part in this tragedy.

Coming in as the first unpopular Addison, Kate Walsh won over fans to the point of earning her own spinoff, Private Practice.

Walsh had been bouncing around TV for a while and landed a spot in the L&O episode Navy Blues. As the title implies, it involved a murder of a Navy officer with Walsh as a possible witness.

It turns out she was having an affair with the victim and the cops suspected she killed him when he tried to cut it off.

With icy-blonde hair, Walsh was terrific as her pilot claimed it was self-defense, and shed been set up by the Navy. Even when she was found guilty, she continued her defense.

Its interesting seeing L&O tackle a case belonging to NCIS and Walsh as the villainess.

As the loveable and stunningly unlucky George OMalley, T.R. Knight won over fans who hoped the guy could catch a break.

His bad luck continued in his appearances in the Law & Order world. In the Criminal Intent episode F.P.S., Knight was one of the two bosses of a computer company suspected of killing their female partner.

Knight played mostly the sad-sack, but the cops realized he was far smarter than he seemed and might have been part of the murder.

After leaving Greys, Knight appeared on SVU. Rollins believed shed tracked down a rapist from an old case, and the DNA matched Knights family man. He protested his innocence and Amaro slowly believed him.

In a huge twist, the gang realized the actual perp was the twin brother the man never knew he had. Knight was terrific in both roles as the nice guy and a monster to truly put George behind him.

As the tough-as-nails but warm-hearted Bailey, Chandra Wilson has been with Greys since the start and not likely to leave soon.

Before that, Wilson can boast a trifecta of different L&O roles. In the second season L&O episode Cradle to Grave, she had a big role as a woman accused of being involved in the death of a child she was babysitting. Wilson was subdued in the role to leave the viewers guessing if she was guilty.

Wilson then made a cameo in the SVU episode Waste when the cops were investigating a woman in a coma who was pregnant. Her smart-mouthed nurse would remind fans of Bailey.

The later SVU episode 911 had Wilson showing up as an FBI tech who aided Benson in tracking down a kidnapped girl calling for help.

All three roles showed a different side of Wilson than Greys fans know.

As Meredith Grey, Ellen Pompeo has been the heart and soul of Greys since it began. Thats a far cry from her two L&O appearances.

In the Season 6 episode Savior, Pompeo was the victim of an attack on her family, which her father barely survived. The detectives suspected the man himself was behind this to salvage some lost money. Pompeo refused to believe it, which led to a dramatic courtroom showdown.

Pompeo then appeared in the Season 10 episode Fools For Love. A crime spree led the cops to catch a man who attacked and killed the sister of his girlfriend, played by Pompeo.

At first, she was heartbroken and rocked. But as the investigation continued, the cops began to suspect she might have been deeply involved in the scam.

Pompeos two roles are miles away from the warm-hearted Meredith either as a shell-shocked woman or a secret schemer.

Greys Anatomy seasons 1-17 on Hulu and Netflix. Law & Order, Law & Order: SVU and Law & Order: Criminal Intent on Peacock. Law & Order: Los Angeles on AppleTV.

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10 Greys Anatomy actors you forgot were on Law & Order - Monsters and Critics