Some opted for opulence while others stuck to tradition, and some looked as though they either over thought or put no thought into their head gear at all. Among the best were the devilishly elegant Zara Phillips, Sweden's Crown Princess Victoria, and Queen Sofia and Princess Letizia of Spain.
But there were some who buckled under the pressure, as you will see after the jump.
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Zara Phillips, daughter of Princess Anne, looked stunning in a silvery gray ensemble and a slanted hat with a flower detail on the left. Designed by Phillip Treacy this look is appropriate and chic.
Victoria looked "Posh" indeed (oh, how far she's come since the Spice Girls) in a navy hat by Philip Treacy paired with a dress from her own collection.
At least she's not barefoot?
Joss Stone played it safe in a red suit and beige hat.
Princesses Beatrice and Eugenie
Thank god for Princesses Bea and Eugenie, without them the wedding would have been just a little too proper. Both hats are by Philip Treacy, Eugenie's in blue with a violet rose to match her Vivienne Westwood suit and Bea's in a lovely shade of rose to match her Valentino coat.
That sound you're hearing is the collective "Ummmm, what?" of anyone looking at this photo. Chelsy was pretty much the envy of every girl on the planet, attending as Prince Harry's date, and accessorized her Alberta Ferretti teal suit with a doily. Probably it was an expensive and custom doily, but still.
Royal Step-Mum Camilla Parker-Bowles wore a traditional ivory hat to match her ice blue and gold look.
Mother-of-the-Bride, Carole Middleton, styled her light blue dress and coat with a hat by milliner Jane Corbett. It's easy to see where Kate gets her style and her looks from.
Princess Marie-Chantal of Greece
Princesss Marie-Chantal has princess down-pat wearing a silvery hat adorned with roses by Philip Treacy.
Wife of Deputy Prime Minister, Nick Clegg, Ms. Gonzalez channels Dr. Lisa Airan at a Copacabana themed Met Ball. Except Dr. Airan would be in Rodarte.
The British socialite wears a fascinator by Philip Treacy (inspired by Georgia O'Keefe?) that matches her bright cobalt look.
Queen Sofia and Princess Letizia of Spain
Queen Sofía and Princess Letizia of Spain looked ravishing in lilac and blush ensembles. The Queen's fascinator mimics the floral embellishment on her jacket, while the princess chose an interesting cloche-like hat that enhances her elegant features.
Princess Letizia of Spain
And a close-up of Spain's Princess.
Princess Victoria of Sweden
Princess Victoria went for a matchy-matchy look in all peach and chose a sophisticated wide brim hat.
Lady Frederick Windsor
Also sporting the wide brim trend is Lady Frederick of Windsor, who paired a chic navy straw hat that kinda looks like a flying saucer with a checked navy suit.
Princess Anne chose a green and purple ensemble topped off with a purple bowler-like hat. It's a lot of florals, but she'll get a pass for embodying that British brand of kitsch.
Queen Margrethe II of Denmark
It's obvious that strict matching was the preferred dress code for many guests, and Queen Margrethe of Denmark follows suit in a full shocking teal ensemble. The Queen wears a tiny chapeau with swirling feathers. Very Seussian.
Lorna Brooking is apparently the wife of a former footballer, but I can only imagine what Michael Kors in Project Runway judge mode would have to say about this one. Or really anyone with any fashion sense.
Queen Elizabeth II
The Queen herself chose a yellow topper with floral touches to compliment her yellow suit. So cheery!
Sir Elton John and David Furnish