Law & Order: Special Victims Unithas kept us on our toes for 17 seasons, and by now, the procedural drama is as well-known for its shocking plot twists as it is for its revolving door of celebrity guest stars. We already know that season 18—which premieres tonight at 9 p.m. ET on NBC—will feature a cameo by Vice President Joe Biden, and we're anxiously waiting to find out which other familiar faces will be walking into the precinct this year.

To celebrate the return of all-new episodes, we're taking a look back at some of the most memorable guest stars ever to grace our screens during an SVU marathon—some of them on multiple occasions, like Modern Family star Sarah Hyland. The 25-year-old actress has appeared on the series twice, first portraying a child victim in the season three episode "Repression" and later returning to the series in the role of a killer in season ten's "Hothouse." And for Hyland, each experience was unforgettable in its own way.

"The first time I was ever on the show was when I was 10, and Mariska Hargitay was just an absolute joy to work with," Hyland told InStyle this week. "My first memory of the set is actually a funny little anecdote. I was walking into the police station to get interviewed, and on the second take, I went up to the director and said, 'Hi, I'm sorry, but my character's mom says the line that we had to come straight from school—but I don't have a backpack. Shouldn't I have a backpack?' They were like, 'Oh my gosh, good catch—thanks, Sarah!' I was 10, and I just figured that I would have to put my backpack down somewhere if I came straight from school. I don't think the props guy was very happy with me for calling him out."

In 2009, Hyland returned to the show to portray Jennifer Banks, a brilliant but sleep-deprived student who killed her roommate in what's arguably one of the most disturbing plot lines to date. "It was really one of my favorite characters I've ever played," Hyland said. "I was obsessed with Girl, Interrupted and had read the book for the first time when I was 15 years old, and I just always wanted to play a character like that. Even when I auditioned for it, I was like, 'This is my Girl, Interrupted moment, and I'm going to try and bring as much justice to it as possible.'" But playing such a mentally unstable character wasn't easy. "The audition process was really grueling," said Hyland. "Plus, I had just turned 18 and it was the first time I was on set without a guardian. So not only was it one of the most demanding roles I had ever played, but I didn't have my mom there. I was all on my own for the first time for that role in many ways—and it was a lot of fun."

Scroll down for more of the most memorable—and surprising—guest stars ever on Law & Order: Special Victims Unit.

Hayden Panettiere

Much like Sarah Hyland, Hayden Panettiere did two stints on SVU: first as a victim, and later as a criminal.In season two, she starred as Ashley Austin Black, the neglected and often-injured daughter of a famous musician in the "Abuse" episode. Panettiere returned in season six, this time playing the manipulative best friend of a murdered high school prostitute in "Hooked."

Sarah Paulson

Sarah Paulson made her acting debut on an episode of Law & Order in 1994, so it's no surprise that she returned to the series' spinoff for another guest role in 2010. She played Anne Gillette, the duplicitous daughter of a wealthy murdered couple in season eleven's "Shadow" episode. After claiming that she was being stalked, the detectives realized that Anne was the one playing them from the start and had killed her parents for money.

Bradley Cooper

Bradley Cooper went from charming to slimy real quick when playing money-hungry defense attorney Jason Whittaker in the season six episode "Night." When his client was accused of raping illegal immigrants, Whittaker covered for the perp, who came from a wealthy family that paid him well.

Abigail Breslin

In one of the weirdest story lines to date, Abigail Breslin starred as 6-year-old Patty Branson in season five's "Birthright" episode. After she was almost abducted in Central Park, the detectives learned that Patty's would-be abductors were actually private investigators hired by a woman who claimed to be the child's biological mother—despite the fact that Patty wasn't adopted. A shocking revelation led to the realization that a sketchy fertility doctor implanted the woman's embryo into Patty's mom—and other patients—without their consent.

Amanda Seyfried

In season six, Amanda Seyfried played the role of rape victim Tandi McCain in "Outcry." After initially falsely accusing army cadet students of the assault, Benson and Stabler suspected that Tandi's stepfather was the culprit. But in the end, it's revealed that her stepfather's boss was actually the one who assaulted her—and even after he's found not guilty at trial, there's hope for justice when another victim comes forward.

John Stamos

Uncle Jesse had serious baby fever in season twelve's "Bang" episode. Full House star John Stamos's role as respectable lawyer Ken Turner took a twisted turn when it was revealed that his character was a reproductive abuser and had fathered children with over twenty women. Ultimately, Ken's actions caught up to him when the detectives' key expert on the subject murdered the sleazeball in cold blood.

Bob Saget

Danny Tanner, is that you? Full House star Bob Saget portrayed the downright frightening Glenn Cheales in season eight's "Choreographed." Suspicious of his wife's fidelity, Glenn closely followed her every move—which led him to the discovery that she was having an affair. Glenn then murdered his wife's lover's wife, a crime that couldn't be masked no matter how hard he tried to come off as the good guy.

Ian Somerhalder

Ian Somerhalder amped up the crazy in the "Dominance" episode of season four. He played Charlie Baker, a manipulative psychopath who raped his younger brother and beat their father. After forcing his brother to go on a killing spree and framing him for additional abductions, Charlie was convicted of his atrocious crimes and faced execution.

Elle Fanning

Elle Fanning played Eden, a young girl found in a van that had been driven into the river in season eight's "Cage" episode. The case led to a subsequent deep-dive into the attachment therapy technique used by Eden's foster parents, which involved keeping the children in cages when they misbehaved. We really felt for Eden—that is, until she set fire to the apartment of Detective Dani Beck, who was only trying to help her. Talk about creepy.

Milo Ventimiglia

Far from Stars Hollow, Gilmore Girls star Milo Ventimiglia played victim Lee Healy in the season five episode titled "Escape." Lee was sexually abused by his mom's boyfriend as a kid, and when the convict escapes from prison, he holds the poor kid hostage while trying to prove his innocence. Ultimately, Benson discovers that Lee was abused by a biological relative—his cousin—instead, but was too ashamed to tell anyone.

Elizabeth Banks

Funny lady Elizabeth Banks took a dark turn for her role in season three of SVU. In "Sacrifice," she played Jaina Jansen, a porn star who was working alongside her husband (played by Mark-Paul Gosselaar, in perhaps the least Zack Morris-reminiscent role of all time) to pay for their sick daughter's medical care. In the end, Jaina turned out to be little more than a killer who was willing to step on anybody that got in the way of her limelight.

Robin Williams

Robin Williams played an audio engineer in one of the show's most chilling guest roles of all time. The season nine "Authority" episode followed his character Merritt Rook, who played mind games with detectives and even represented himself when on trial for conspiracy to commit sexual assault (he won the case). Merritt didn't believe in authority, but he was the one who wanted the control. When Benson and Stabler realized that he was responsible for the death of a doctor who failed to save the lives of Merritt's wife and unborn child, they set out to arrest him—only Merritt kidnaps Benson instead. After playing a terrifying game of cat and mouse with Stabler, Merritt released Benson and revealed that his threats were nothing more than smokescreens. He ultimately escaped, and left the partners feeling more grateful for each other than ever before.

Cynthia Nixon

Leaving the glamorous version of Manhattan from Sex and the City behind, Cynthia Nixon took on a much grittier character in "Alternate" in season nine. She played Janis Donovan, a woman who appears to have recently abused her young daughter. Only as it turned out, she didn't have a daughter—instead, the detectives discovered that Janis suffered from suffer from Dissociative Identity Disorder and had four alternate personalities. Ultimately though, the joke was on Stabler and Benson. Janis was competent all along, and she faked her mental instability as part of a master plan she and her sister created to murder their parents. Miranda Hobbes would not approve.

Rooney Mara

Way before she was The Girl with the Dragon Tattoo, Rooney Mara made her acting debut as a bully-turned-victim in the season seven episode titled "Fat." Her 16-year-old character, Jessica DeLay, beat another student up for being overweight—and was later assaulted by her target's younger siblings as retribution for her actions. Fun fact: Mara's sister, Kate, also appeared on SVU as a suspect teen gymnast in the season two "Pixies" episode.

Hilary Duff

In season ten's "Selfish" episode, Hilary Duff portrayed a young mom named Ashlee Walker. When her daughter went missing, Ashlee's irresponsible behavior was called into question—and SVU was convinced that she murdered poor 11-month-old Sierra to continue partying without having to care for her child. But the episode took a strange turn when Sierra's body was found, and it was revealed that her cause of death was actually the measles. From there, the story delved into the controversy surrounding childhood vaccinations.

Zoe Saldana

In the season five episode "Criminal," Zoe Saldana portrayed Gabrielle Vega, the daughter of a convicted murderer. The SVU squad discovered that Gabrielle's father had been framed for murder when he was 19—before he even knew that his daughter existed—but after years spent behind bars, he had nothing left of his former life. When he returned to using drugs, Gabrielle filed a lawsuit against the NYPD seeking reparation.

James Van Der Beek

James Van Der Beek's portrayal of predatory Sean Albert in season thirteen's "Father Dearest" quickly erased all memories of love-struck Dawson Leery from the Creek. Sean lured teenage girls who were searching for their biological fathers online, tricking them into sleeping with him. He ultimately got caught after seducing the daughter of his ex-girlfriend as revenge for the fact that she dumped him.

Brittany Snow

A pre-Pitch Perfect Brittany Snow played bipolar teen Jamie Hoskins in the season seven episode "Influence." After claiming that she was raped by two classmates, the detectives learned that the encounter was, in fact, consensual—which sent Jamie into a literal tailspin. Off her medication, she tried to kill herself by crashing her car, but ended up taking the life of a pedestrian instead. After trial, she finally managed to get the help she needed.

Jennifer Love Hewitt

In season twelve's "Behave" episode, Jennifer Love Hewitt portrayed Vicki Sayers, a woman who told detectives that she had just been raped for the fourth time in the past 15 years—and all by the same man. At first, there was skepticism about her story—it didn't seem plausible that the same perp had stalked and attacked the same victim without being caught for so long. But in the end, Vicki was vindicated, and she got to slam the prison cell door in her tormentor's face before moving on with her life at last.

Serena Williams

In one of her first acting roles, Serena Williams swapped out her tennis racket for a basketball in season five's "Brotherhood" episode. She made a quick cameo as an athlete who was drunkenly filmed at a college bar and ended up on a dirty website run by a local frat.

Debra Messing

Debra Messing starred as Nancy Grace-esque crime show host Alicia Harding in season twelve's "Pursuit" episode. After dedicating her life's mission to getting justice for her long-ago abducted sister, Alicia began receiving threats. Although she made a few missteps along the way and ignored the advice of detectives to avoid using herself as bait, Alicia ultimately helped the squad catch her sister's killer—although she lost a close friend in A.D.A. Sonya Paxton to the murderer in the process.

Mischa Barton

When Mischa Barton left The O.C., she headed to SVU. The actress took on the role of Gladys Dalton, a pregnant ex-prostitute who witnessed the deaths of two call-girls in season eleven's "Savior" episode. She agreed to testify against the murderous minister responsible for killing prostitutes and leaving prayer cards at his crime scenes, and ended up giving birth prematurely to a severely unhealthy newborn. At the end of the episode, Gladys disappeared and left Benson with the power to make medical decisions for her infant.

Melissa Joan Hart

Melissa Joan Hart starred as high school teacher Sarah Trent in season nine's "Impulsive" episode. When she was accused of raping one of her male students, Shane Mils, Sarah countered that he was the one who assaulted her—which led the detectives to the realization that Shane was suffering from a brain disorder that caused a sex obsession. He went to rehab for the condition with Sarah's blessing, but became a victim of abuse himself while trying to recover at the clinic.