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Remember when these celebrities guest-starred on 'Law & Order: SVU'?

We all love the show for Mariska Hargitay's Olivia Benson, but the steady stream of guest stars truly makes this must-see TV!
/ Source: TODAY

Ever since it debuted in 1999, "Law & Order: SVU" has been the best kind of arresting television — surprising, emotional and uncompromising. And now it's about to air its 400th episode!

Each week, millions tune in to see which baddies Lt. Olivia Benson (Mariska Hargitay) and crew will bust their butts trying to take down each week — but let's face it, we also love to goggle at the stream of famous faces taking their turns as guest stars. One even made a special congratulatory video for the gang!

So in honor of this momentous milestone, we present some of our favorite guests of all time — one of whom went on to have an even bigger starring role, as Hargitay's husband!

Debra Messing

Debra Messing plays Alicia Harding in "Pursuit." NBC

Messing played true-crime series host Alicia Harding, whose sister was murdered years earlier. When the cold case of that murder starts heating up, Alicia taunts the killer on the air and an ADA ends up dead. (Season 12, Episode 17, 2012)

Sarah Hyland

Sarah Hyland guest-starred opposite fellow famous face Amy Irving in "Repression," the first of her two appearances on the show thus far. Eric Liebowitz / NBC

A very young Hyland guest-starred as Lily Ramsey, the sister of a woman who claimed she'd been sexually abused by her father years earlier, and detectives worried about whether the girl should remain in her parents' custody. Hyland became a repeat offender in 2009 when she appeared as a student who kills her classmate in "Hothouse." (Season 3, Episode 1, 2001)

Kyle McLachlan

Kyle MacLachlan (Brett Morton) dispensed his own form of justice in "Conscience," with Christopher Meloni as Detective Elliot Stabler and Diane Neal as ADA Casey Novak.NBC

When his elder son kills his younger son, Brett Morton (MacLachlan) shoots the boy in court because he thinks he's a sociopath. McLachlan returned to the show in 2011's "Blood Brothers." (Season 6, Episode 6, 2004)

Amanda Seyfried

Things got heated in "Outcry" when Tandi (Amanda Seyfried) covered up crucial information. Also pictured: Christopher Meloni as Det. Elliot Stabler and Joseph Lyle Taylor as Jamie BarriganWill Hart / NBC

Claiming she was raped by three men, Tandi McCain (Seyfried, in one of her first non-soap opera roles) turned out to be covering for the real rapist — her father's boss. (Season 6, Episode 5, 2004)

Dana Delany

Everybody's in it for the publicity in "Obscene." Pictured: Dana Delany as Carolyn SpencerWill Hart / NBC

Activist Carolyn Spencer's (Delany) son is arrested for raping a teen starlet, and she blames a radio shock jock for his behavior ... then shoots him to gain support for her cause. (Season 6, Episode 3, 2004)

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Brittany Snow

In "Influence," Jamie Hoskins (Snow) faced off with ADA Casey Novak (Diane Neal). Also pictured: Jill Marie Lawrence as Dr. Cleo ConradWill Hart / NBC

A bipolar teen (Snow) takes her rock star idol's advice and goes off her meds, then mows down pedestrians with her parents' car. The trial then becomes a soapbox for the musician, who is unrepentant about taking medication. (Season 7, Episode 22, 2006)

Elle Fanning

Fanning's performance in "Cage" was particularly heart-rending. Also pictured: Connie Nielsen as Det. Dani BeckWill Hart / NBC

There may be no happy ending for Eden (Fanning) a little girl whose foster parents forced her to sleep in a cage in a barn. (Season 8, Episode 8, 2006)

Nicole "Coco" Austin

So nice when spouses can work together! Austin appeaerd in "Head" as a porn queen alongside husband Ice-T (Det. Odafin "Fin" Tutuola).Will Hart / NBC

Appearing as an adult film star who helps the detectives find a man hiding cameras in women's toilets, Austin — who married "SVU" veteran Ice-T (Det. Odafin Tutuola) in 2002 — made a brief but memorable first appearance on the show. She's appeared two other times: In 2007's "Fights" and 2012's "Theatre Tricks." (Season 5, Episode 25, 2004)

Melissa Joan Hart

Somebody needs help in "Impulsive," where Sarah Trent (Hart) is accused of assaulting one of he rstudents.Craig Blankenhorn / NBC

Accused of raping one of her students, Sarah Trent (Hart) says he in fact raped her, and she believes he's a sex addict who needs help. (Season 9, Episode 3, 2007)

Terrence Howard

Howard played a Los Angeles ADA in "Reparations" opposite Ice-T as Det. Tutuola.Virginia Sherwood / NBC

As Los Angeles ADA Joe Dekker, Howard comes East to defend his cousin, who's been accused of raping a woman in her home in this crossover with the short-lived "Law & Order: LA." (Season 12, Episode 21, 2011)

Elizabeth Mitchell

Something's off-key in "Totem," in which Mitchell appeared as piano teacher June Frye. Also pictured: Mariska Hargitay as Det. BensonWill Hart / NBC

When a piano teacher (Mitchell) confesses to abusing and killing a student, it turns out she's been covering for a loved one. Mitchell is also a repeat offender, from 2003's "Mercy." (Season 12, Episode 20, 2011)

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Jennifer Love Hewitt

In "Behave," Hewitt plays Vicki Sayers, a woman trapped in an unending nightmare.NBC

Vicky Sayers (Hewitt) lives frightened and alone in her home, visited every few years by a man who rapes her. Det. Benson tracks the serial rapist across multiple states to get her man. (Season 12, Episode 3, 2010)

Patricia Arquette

Arquette had to watch her back in "Dreams Deferred."Michael Parmelee / NBC

As a prostitute with a serial killer for a client, Jeannie Kerns (Arquette) agrees to help Det. Benson trap him — but the gambit is not without risk. (Season 14, Episode 9, 2012)

Peter Hermann

Hermann, who first appeared in 2002's "Monogamy," shown here in 2014's "Spring Awakening" with Emma Greenwell as Ellie Porter.Michael Parmelee / NBC

True, Hermann's role as attorney Trevor Langan (who in his first episode is hired to defend Det. Benson when she's framed for murder) is really more of a recurring role than a guest one, but who could resist, knowing that his frequent appearances led to sparks with Hargitay? The pair wed in 2004 and have three children. (Season 3, Episode 11, 2002)

Alec Baldwin

Hargitay directed and starred opposite Baldwin in "Criminal Stories," where he played a shady wordsmith. Michael Parmelee / NBC

Baldwin, known for scrapping with the press in real life, plays an infamous New York Ledger reporter assigned to write a feature on Det. Benson and the Special Victims squad, but he may be far from the objective outsider he seems at first. (Season 15, Episode 18, 2014)

Sophia Bush

Sometimes it takes two to make a case go right: Here, Bush (as Det. Lindsay) appears in "Daydream Believer" alongside Hargitay as Det. Benson.David Giesbrecht / NBC

Bush first appeared as Det. Erin Lindsay, brought over from "Chicago P.D." in 2014 and has since made four appearances. In her first episode she worked with Det. Benson to take down a child pornography ring and catch a serial killer leaving victims in both their cities.

"Law & Order: SVU" airs Wednesdays at 9 p.m. ET on NBC.

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