Princess Eugenie is marrying her long-term partner Jack Brooksbank at St George’s Chapel in Windsor.
The Royal Family and celebrities are among the 850 guests at the ceremony.
The ninth in line to the throne is being given away by her father, Prince Andrew – watched by her grandmother, the Queen.
Prince Philip, Eugenie’s mother, the Duchess of York, and her sister Beatrice, are also attending, along with crowds of well-wishers outside.
Around 1,200 members of the public selected by ballot are listening to a live broadcast in the castle grounds, and thousands are watching outside in extremely windy conditions.
The wedding is being screened in full on ITV as part of a three-hour special of the daily magazine show, This Morning.
The 28-year-old and drinks executive Jack Brooksbank, 32, are tying the knot at St George’s Chapel – where Prince Harry and Meghan Markle married in May.
The couple met when skiing in Switzerland in 2010.
The Duke and Duchess of Cambridge’s children, Prince George and Princess Charlotte, are taking on pageboy and bridesmaid roles.
The other bridesmaids are Mia Tindall – daughter of Zara and Mike Tindall – Mia’s cousins Savannah and Isla, Eugenie’s goddaughter Maud Windsor and Robbie Williams’ daughter, Theodora.
The other pageboy is Louis de Givenchy, six, the son of friends.
Eugenie’s sister, Princess Beatrice, is maid of honour. Mr Brooksbank has chosen his brother, Thomas, as best man.
Among the celebrities sharing their special day are singer James Blunt and his wife Sofia Wellesley, singer Robbie Williams, model Pixie Geldof, comedian Jimmy Carr and David Emanuel – who designed Princess Diana’s wedding dress.