The massed bands of the Household Division will provide musical backing during the day and also taking part is the King's Troop Royal Horse Artillery who, following the parade, will fire a 41-gun salute in Green Park to mark the Queen's official birthday.
Click through the gallery to see the most stylish looks donned by the royals at 2018's Trooping the Colour.
As the entire British monarchy stood on the balcony of Buckingham Palace on Saturday to celebrate the Queen's birthday parade, one tiny member was noticeably missing from the festivities. Kate, who is married to Harry's older brother Prince William, wore a dress by Alexander McQueen and a hat by Juliette Botterill.
Stylish as ever, she opted for a "pretty in pink" vibe with a sophisticated off-the-shoulder pastel dress by Carolina Herrera - a favourite designer among the royals.
The entire royal family gathered together on the balcony to watch the iconic fly-past by the Royal Air Force. Meghan's already looking fully at ease. laughing and smiling with her father-in-law and Kate.
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The members of the royal family - yes, including now Meghan Markle, the Duchess of Sussex - stepped out on Saturday in London for the annual Trooping the Colour parade, the official celebration of Queen Elizabeth II's birthday.
While Meghan has blossomed into the epitome of royal sophistication already, Prince Harry in his black uniform looked like a knight in shining armour. She is due to join Queen Elizabeth in Cheshire for her first solo appearance alongside the Queen on Thursday.
The view along the Mall heading back to Buckingham Palace.
Meghan also had her hair styled in a bouncy blowout. That Meghan is undertaking such a trip less than a month after marriage says a lot about how the family wants to make sure she's comfortable in her duties, and that the world knows that she is now an important full-time royal.