Austin Butler, Kaia Gerber, Presley Gerber, Cindy Crawford, and Rande Gerber

At first glance, I thought Kaia was LISA RINNA. But given that Austin is Andy Warhol, I assume she is Edie Sedgwick. Cindy and Rande are obviously Sandy and Danny. Presley is...I have no idea. He spent a lot of time with Bieber, who was wearing a snorkel. Are they human Spongebob characters?

Justin Bieber and Hailey Bieber

He and Hailey also went out as Flintstone characters; I always think costumes like Hailey's look like they might feel HUMID.

Shaun White, Nina Dobrev, Miles Teller and Keleigh Sperry

Shaun White and Nina Dobrev's Axel Rose and Slash are GREAT; Miles Teller and Keleigh Sperry are....again, you got me!!

Megan Fox and Machine Gun Kelly

Megan actually tagged SAG in her Instagram post of this look -- they're from Kill Bill -- which seems weirdly combative to her own union! I will say that she looks great with this hair.

Machine Gun Kelly and Megan Fox

These costumes are from a manga called DEATH NOTE (which has been adapted a bunch of times).

Jodie Turner-Smith

Is she...a cute little giraffe??? (Judging from the pattern.)

Paris Jackson

Is this...one of Robin Hood's merry men? I enjoy her backpack.

Paul Wesley and Natalie Kuckenburg

Classic sexy cat, accompanied by...I dunno? A DUDE?

Darren Criss

"Oh, I actually was just wearing this."

Victoria Justice

Okay but this Cher costume is RAD. Cheers to Victoria Justice.

Paris Hilton

...is Paris a Sexy Mushroom from Super Mario Bros?

Ashley Benson

THIS is the classic costume, Last Minute Vampire.

Courtney Reum, Paris Hilton, and Carter Reum

I swear to God, Paris has worn this Britney costume before -- they're all the Toxic video. Paris is probably riding high on the fact that she's one of the only people who comes out of Britney's memoir unscathed.

Kelly Sawyer Patricof and Jessica Alba

Speaking of Britneys! Being the Naked Toxic Britney is the move of a woman who has a lot of body confidence and good for her!

Halsey

I believe this is a reference to one of their own songs, but also a classic Creepy Mermaid look.

Dylan Mulvaney

Snow White Number One....

Demi Lovato

....meet Snow White the second.

Sabrina Carpenter

Ah, yes! Your classic Sexy...um...Princess???

Kelly Ripa and Mark Consuelos

The workmanship here is objectively EXCELLENT.

Myles Frost

Instead of MJ -- the role he played on Broadway -- his shirt says MF. Is he dressed as himself in his Tony-winning role? Because that's funny.

Padma Lakshmi

Aha! Behold a Big Bad Wolf.

Chloe Bailey

Catwoman is a classic, but she does need her whip!

Shauna Robertson and Edward Norton

...Ed Norton DOES kinda look like David Beckham????

Rachel Lindsay

She is, obviously, Rihanna at the Super Bowl and this is really cute. And probably comfy?

Sara Sampaio

I haven't seen a classic Flasher costume in...a long time.

Ashley Madekwe

Ashley Madekwe is in the Matrix!

Gavin Rossdale (L) and guest

Gavin Rossdale looks like he's in costume as himself. His date looks...kinda like Lady Gaga?

Lauren Conrad (L) and guests

When in doubt...

Skeet Ulrich (R) and guest

So we've got a...sexy bride??? And, of course, Willie Nelson.

Molly Sims, Scott Stuber, and guest

I DO love Gwen Stefani's cameo in Scarface.

Alessandra Ambrosio

I hope she, too, is basically in costume as herself: An iconic Victoria's Secret model.

Brett Oppenheim and Jason Oppenheim

Astronaut and cowboy is actually very wholesome.

Chord Overstreet, Glen Powell, Sarah Hyland and Wells Adams

I assume Chord and Glen are Talladega Nights -- a movie I ALWAYS forget Amy Adams is in -- and Wells is....a hockey player, and Sarah is....Kate Gosselin? (No but I like the concept.)

This Weekend Was Hollywood's Halloween

And probably party weekend for a lot of you!