As with Harry and Meghan's wedding, 1,200 members of the public have been given tickets to hear a broadcast relay of the ceremony from outside the chapel doors.
The royal family have also arrived.
One noticeable absentee was Camilla, the Duchess of Cornwall, the wife of heir-to-the-throne Prince Charles, as she was carrying out an engagement in Scotland.
It fell to the floor, and revealed a glimpse at her back.
It's royal wedding eve, you guys, and if you're anything like us, the anticipation surrounding Princess Eugenie's nuptials at Windsor Castle on October 12 (aka tomorrow) is tough to bear.
At the Duke and Duchess of Sussex's wedding in May, the 36-year-old wore an ivory Alexander McQueen design which she had previously worn three times before.
After the service, the couple will embark on an open-top carriage tour of Windsor, although shorter than the one Harry and Meghan, now the Duke and Duchess of Sussex, took when thousands lined the streets of the town.
The bride's mother, Sarah Ferguson, has also taken up a seat in the church dressed in green, alongside her oldest daughter Princess Beatrice, who is also serving as her sister's maid-of-honour. But, I think that for visitors, the first wedding was such a big occasion.
The couple have also confirmed their bridal party and you'll recognize some adorable familiar faces including bridesmaids Princess Charlotte, Savannah Philips, Isla Philips and Mia Tindall.
The whole wedding will be shown on a This Morning special.
Buckingham Palace released new details Thursday about Princess Eugenie's upcoming wedding, including the Order of Service and the charities invited.
And with the presence of so many royals - and a number of celebrity guests - extra security measures have been put in place.