Guests including the Duke and Duchess of Cambridge are attending the wedding of Pippa Middleton and James Mathews.
Miss Middleton, 33, is the younger sister of the duchess, and got engaged to 41-year-old Mr Matthews last year.
The private wedding ceremony at St Mark's Church in Englefield, Berkshire, got under way at 11:30 BST (10:30 GMT).
The church is a few miles from the Middleton family home in Bucklebury and on private land - and is relatively sheltered from the public and media.
It is understood strict security is in place on the estate, with villagers being asked to carry ID on the day, and asked not to speak to the press.
The Duke of Cambridge, Prince Harry, Princess Beatrice and Roger Federer and his wife Mirka are among the guests to have arrived at the church.
The Duke and Duchess of Cambridge and Prince Harry are on the guest list.
The bride's nephew and niece, Prince George and Princess Charlotte, will be page boy and flower girl.
Actress Meghan Markle, Prince Harry's girlfriend, is also reportedly in the UK to attend the event.
The best man is reality TV star Spencer Matthews, the groom's 28-year-old younger brother.
Miss Middleton's diamond engagement ring is reported to have cost £250,000 - nearly 10 times the budget for the average wedding day.
But details of the couple's wedding bands have not been revealed.
Guests will celebrate the marriage in a large glass marquee, erected on the Middleton family's property.
Among the members of the public who have gathered in the village is Meryl Elizabeth Evans, from south Wales.
"It's a nice atmosphere, everybody's friendly, it's just a nice country feel to it," she said.
"We've met people from around the world, and the camera crews are from everywhere. It is the wedding of the year, isn't it?"