Madonna
For every great outfit at one of these events, there are a million bad ones and last night proved to be no exception to the rule. We honestly had to try and whittle down the worst dressed to just three main contenders!
Why oh why celebs get it so wrong is beyond us, after all they have a team of stylists, hairdressers and make-up artists on hand to make them look stunning, and yet they still end up looking like a bag lady.
It's a toughie to pick who will get slated first here, as arguably that signals which outfit was the worst and trust us, when you see the looks we've picked out, you'll understand our dilemma.
Ok, here goes- Madonna you are officially awarded the title of worst dressed at this stylish event, and you should hang your head in shame for this pitiful Puss-in-boots inspired outfit.
My god, everything about this outfit is wrong; the chavvy thigh-high hooker boots, the bizarre blue head scarf and the dodgy leather cuffs just scream cheap, although we bet they cost a fortune.
Her headpiece reminds us of Carrie's blue bird she put on for her wedding in Sex and The City, and we all know what a bad choice that ended up being in the movie- Madge, what were you thinking?!
To give her some credit, her body looks in terrific shape and she must have found the fountain of youth, because her skin was wrinkle free and glowing on the night.
But Madge, you are 50 and wearing a puffy blue mini-dress that goes right up your bum, complete with thigh-high boots is sooo not a good look for you; why not try a full-length couture gown for next year!?
It seemed that taking leave of your style senses was a common theme on the night, as we were left opened mouthed by Gossip Girl star, Leighton Meester's outfit for the occasion.
Leighton Meester at the Costume Institute Ball
Considering we championed her as a style icon late last year, and she's so fashion forward as Blair on the show, we take it as a personal insult for how bad she looks here.
A crazy puff sleeved pant suit with bow shoes is just oh-so wrong, and is a look that should never of been allowed to have been aired in public.
Leighton is gorgeous and has the benefit of youth on her side, but when you pair this disastrous number with ageing curly hair and OTT make-up, you end looking like your in your late 30s, like she unfortunately looks here.
We're also hating the new honey blonde hair colour, so please ditch the bleach and go back to that gorgeous brown colour you've become famous for, and while your at it opt for nude lippy and less blusher next time, and you'll be back at the top of our fashion hotlist in no time.
Finally, we have to despair at Rachel Weisz's horrendous look on the night, as she sampled the black lipstick trend straight off the catwalk and ended up looking like a vampire.
Rachel Weisz at the Costume Institute Ball
Poor Rachel, you could kind of see where she was going with her whole look, but it just ended up ticking every box for being mumsy and old fashioned on the night.
The pale pink frock might have worked if the fishtail-like skirt hadn't been added three-quarters of the way down, and then messed up even further with a hideous mustard coloured underlining.
Her accessories were mismatching and her hair was too messy to be considered chic for such a prestigious event, not to mention her eye and cheek make-up looked like they had been done in the dark. Not your finest hour Ms.Weisz.
FemaleFirst- Laura Terry