Janis Ian mastered the artsy grunge look.
Janis Ian's angsty aesthetic proved she was the complete opposite of the Plastics.
Her most iconic outfit included a "Rubbish" shirt over a long-sleeved tee. She completed the look with a black skirt over striped leggings and high-top Converse sneakers.
The Plastics all made some alterations to their gym uniforms.
High-school gym uniforms are notoriously unflattering, but the Plastics managed to bring a 2000s, sultry flair to their looks.
Regina fashioned a skort out of her short gym shorts and paired it with a tight T-shirt.
Gretchen bared a lot of midriff by tying up her gym shirt and pairing it with low-rise drawstring shorts.
Karen cut the sleeves off of her tight tee to fashion a tank top and paired it with super-short shorts, frilly white socks, and pink and white sneakers.
The Plastics welcomed Cady in her first "pink Wednesday" look.
Unprepared for all of the Plastics' rules, including wearing pink on Wednesdays, Cady resorted to borrowing Damian's Lacoste polo.
She completed the look with a pair of flared jeans and sneakers.
This is a major contrast to the Plastics, who donned high heels, miniskirts, and small Louis Vuitton handbags that screamed early-2000s fashion.
Regina's mom wore her "cool mom" Juicy Couture sweatsuit.
When the Plastics head back to Regina George's mansion after shopping, Cady is introduced to Regina's mom who is wearing an age-inappropriate pink Juicy Couture sweatsuit with a lace-embellished white tank top underneath.
Juicy Couture's velour tracksuits were ubiquitous by 2004, but it was the way Poehler played the character and abided by the Plastics' pink on Wednesdays rule that made this look so iconic.
Cady's first strut down the hallway as a Plastic was the ultimate 2000s style statement.
Cady traded in her drab flannels for high heels and miniskirts to blend in with the Plastics.
After getting a makeover from her new friends, Cady walked down the school hallway wearing a pastel plaid miniskirt paired with a coordinating tank top and jacket. She completed the look with a colorful Louis Vuitton purse and pink pumps.
Regina wore a glam pajama set when she orchestrated the three-way call attack on Cady.
Most people wear sweats or T-shirts to bed, but not Regina George. The leader of the Plastics looks chic in a monogrammed pink pajama set.
And of course, she can't forget the sparkly "R" necklace, an accessory she's seen wearing throughout the film.
Gretchen put an edgy spin on her usual preppy style in a Burberry miniskirt and "royal punk" tee.
Though the girls often dressed in coordinated looks, they each had standout style moments throughout the movie.
Gretchen Wieners' best look was a red "royal punk" slogan tee paired with a belted Burberry pleated miniskirt. The top was tied in the back to show off some midriff and give the outfit a bit of edge.
The movie's costume designer, Mary Jane Fort, told MTV that when it came to styling Gretchen, she "went back to a more nostalgic time, almost a '50s style, with her little skirts and kept her not quite as provocative as Regina."
Regina started a new fashion trend after Janis cuts two holes in her shirt.
Although it's not a trend the average person would rock, Regina George made a statement by shamelessly wearing her tank top after Janis cut it up.
The queen bee completed the unique look with a purple bra, black miniskirt, and black pumps.
The Plastics' Halloween costumes were equally over-the-top, including Regina's bunny look.
Much to the chagrin of her father, Regina wore a sparkly silver bunny outfit with white furry detailing, knee-high white satin boots, and of course, pink bunny ears.
Fort told Nylon that the Halloween looks were her favorite, adding that "Rachel [McAdams] as the bunny was the most intense."
Gretchen Wieners wore a skintight catsuit.
Dressed as Catwoman, Gretchen showed up to the Halloween party wearing an all-black outfit with small cat ears.
She completed the look with a pair of black high heels.
And Karen Smith dressed as a mouse, duh.
Although it wasn't obvious to anyone else, Karen dressed as a mouse. She accessorized her frilly black slip with a pair of fuzzy ears.
Cady went all out for Halloween and dressed as an "ex-wife."
Unlike the rest of the Plastics, Cady didn't appear to get the sexy costume memo.
Instead, she opted for a scarier look.
Dressed as a zombie "ex-wife," Cady wore a wedding gown splattered with blood, a long black wig, and a set of unflattering teeth.
The Plastics pushed the envelope at the talent show in their Santa outfits.
For their annual talent show performance of "Jingle Bell Rock," the Plastics wore coordinating outfits complete with bedazzled Santa hats, red leather miniskirts, black knee-high boots, and black gloves.
Fort told Nylon that she didn't want the Santa outfits to "be trashy," adding that "there was a little element of some sort of taste."
The iconic Santa outfits are also one of the "Mean Girls"-inspired looks Ariana Grande recreated in her "thank u, next" music video.
Cady started to adopt Regina George's style after becoming the new queen bee.
During a get-together at her house, Cady wore a strapless minidress with sheer tights and a black choker.
Sticking to the early-2000s trend, she decided to show off her bright-pink bra straps instead of opting for a strapless style.
Regina sported her initial necklace again to spread the Burn Book pages across the entire school.
During her quest for revenge, Regina was seen wearing a black off-the-shoulder top with exposed baby blue bra straps, a fashion trend that was prominent in the early 2000s.
She paired her top and flared denim with a matching pink Louis Vuitton belt and purse.
Damian's incognito look is a classic.
Although it's not the most fashionable look from the film, Damian's blue hoodie and black sunglasses are still iconic.
Daniel Franzese, who played Damian in the movie, told Cosmopolitan the "She doesn't even go here" scene was one of his favorites in the film.
"I'm covered up, wearing sunglasses with a hood, and they were worried I wouldn't be recognizable enough, but I was a huge guy standing in a crowd of all girls. I had to negotiate that," he said.
Karen wanted to add a little more bling to her Spring Fling look, but the mirror got the best of her.
For Spring Fling, Karen chose to jazz up her brown and blue paisley slip dress with a stick-on bedazzled "K" on her chest. Unfortunately, she lived up to her reputation by accidentally sticking it on backward in front of the mirror.
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