Also attending are Prince Charles and Camilla, Prince Harry and Meghan, the Duchess of Cambridge's parents, her brother and sister and Louis's six godparents, who are close friends or relatives of William and Kate.
We can't wait to see more photos! Her new sister-in-law, the Duchess of Cambridge, wore an ice blue fitted dress and matching fascinator.
A video of Prince William and Duchess Catherine, both 36, and their brood entering the chapel was shared on the Kensington Palace Twitter account, which shows Catherine holding their youngest tot in her arms whilst he sleeps.
After the brief speech, the Queen joined the rest of the royals up on the palace balcony to watch nearly 100 aircraft, from Second World War Spitfires to the newest stealth fighter jet, fly over the palace.
This is the second official flypast in two month as the royals celebrated the Trooping the Colour in honor of Queen Elizabeth's birthday in early June.
She also took the Royal Salute as the Prince of Wales and the Duchess of Cornwall, the Duke and Duchess of Cambridge and the Duke and Duchess of Sussex watched.
Eagle-eyed watchers have noted that Meghan, who was wearing a black Dior dress with bateau neckline, was missing a brooch in her outfit.
And despite still being on maternity leave, after giving birth to Prince Louis only 11 weeks ago, it is understood that Kate felt it was important to attend Tuesday's celebrations.
Little Princess Charlotte may be just 3 years old, but she's already a royal scene-stealer.