The 92-year-old monarch has a busy week ahead, marked by a visit from US President Donald Trump in Windsor on Friday. Speaking at Westminster Abbey, the Archbishop of Canterbury, the Most Reverend Justin Welby, said the RAF had played a "decisive factor in saving this country's independence, its democracy and its freedom; its hope of civilization and its contribution to humanity for the future".
As you may be aware, Prince Louis was christened yesterday and the royal event was the source of much media attention.
She also shared a handshake with The Dean of The Chapel Royal, The Right Reverend and Right Honourable the Lord Chartres before heading inside.
Meghan wore a long-sleeved, full-skirted black Dior dress with a bateau neckline, topped off with a Stephen Jones fascinator.
Louis will wear the handmade replica of the Royal Christening Robe, which in 2008 replaced the original which had been worn for all royal christenings since that of Queen Victoria's eldest daughter in 1841.
Kate's parents and siblings were also there, including her sister Pippa Middleton, who is pregnant.
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The Duke and Duchess of Cambridge sitting in Westminster Abbey waiting for the commencement of a ceremony as part of the RAF Centenary celebrations.
Prince Harry will certainly do the same if - or perhaps when - he has children with new wife Meghan Markle. Guests will be served slices of christening cake, which is a tier taken from The Duke and Duchess of Cambridge's wedding cake, which was designed by Fiona Cairns.
A photo shared by People showed Charlotte poking out her tongue while George could be seen next to her with his mouth agape.
It was a family affair for the Middletons, who all stepped out on Monday afternoon to celebrate Prince Louis' christening.
It's likely that Louis will have five godparents, as does Princess Charlotte, whereas Prince George, a future king, has seven.