Millions of people around the world are set to watch the wedding of Prince Harry and Meghan Markle in Windsor.
Hundreds more have spent a night on the streets of Windsor to secure the best viewing spot.
The couple will wed at noon at St George's Chapel in Windsor Castle, before travelling through the town in a carriage at about 13:00 BST.
In her vows, Ms Markle will not promise to "obey" her husband and Prince Harry has chosen to have a wedding ring.
Prince Charles will walk Ms Markle down the aisle, after her father, Thomas, was unable to attend for health reasons.
On the eve of their wedding, Prince Harry told crowds in Windsor he was feeling "relaxed" and Ms Markle said she was feeling "wonderful".
Analysis by BBC royal correspondent Jonny Dymond
Every royal wedding is different. But every royal wedding is an opportunity, in some way, to relaunch the Royal Family.
Big weddings like this come along pretty rarely and they are now the object of global fascination.
So it represents a great opportunity to say: "This is who the Royal Family are these days."
But this is a very different royal wedding.
It's different because of the style of the arrangements for the day itself.
From small things, like the cake (not a traditional big heavy fruitcake covered with bullet-proof icing), to bigger things, like a gospel choir performing at the service.
To more remarkable decisions, like the invitation to 1,200 members of the public to enjoy the occasion in the grounds of Windsor Castle.
After the death in 1997 of Prince Harry's mother, Diana, Princess of Wales, she was described by the-then prime minister Tony Blair as "the people's princess".
This may not be "the people's wedding", but it is about as close to it as any royal wedding has got.
Read more here.
Crowds of well-wishers - many wearing union flags - royal superfans and the world's media have been gathering in Windsor in the week leading up to the big day.
As many as 100,000 people are expected to line the streets, hoping to get a glimpse of the happy couple.
Nicky and Matt Pruner, from the US, extended their holiday in the UK to be in Windsor for the wedding.
Nicky Pruner, 62, said: "I camped out all night at Kate and William's wedding and I said I would never do it again - and here I am."
Matt Pruner, 63, added: "It feels like a festival, except this one has a meaning. It just gives hope to the world."
Six hundred guests will watch the wedding, officiated by the Archbishop of Canterbury, Justin Welby, in St George's chapel, with the service broadcast to millions across the world.
About 1,200 members of the public - many who are recognised for their charity work - have been invited into the grounds of Windsor Castle for the wedding.
Prince Harry is expected to arrive with his brother and best man Prince William at the west door of the chapel at 11:40.
The Queen will be the last member of the Royal Family to arrive at 11:52, before Ms Markle arrives with her mother Doria Ragland.
Ms Markle has 10 bridesmaids and pageboys, all under the age of eight - including Prince George and Princess Charlotte.
The bride will be met by Prince Charles, who will walk her down the aisle of the Quire of the chapel.
During the service, the couple will pledge themselves to one another, saying: "For better, for worse, for richer, for poorer, in sickness and in health, to love and to cherish, till death us do part."
The Most Rev Bishop Michael Curry, the 27th presiding bishop and primate of the Episcopal Church, from Chicago, will give an address at the wedding and the Rt Rev David Conner, Dean of Windsor, will conduct the service.
Lady Jane Fellowes, the sister of Prince Harry's late mother, Diana, Princess of Wales, will deliver a reading.
As the bride and groom sign the register, 19-year-old cellist Sheku Kanneh-Mason - who won the 2016 BBC's Young Musician - will perform three pieces - by Faure, Schubert and Maria Theresia von Paradis, with musicians from the BBC National Orchestra of Wales, the English Chamber Orchestra and the Philharmonia.
Karen Gibson and The Kingdom Choir will perform Ben E. King's soul classic Stand By Me during the service.
The gospel choir will also perform Etta James' uplifting version of Amen/This Little Light of Mine as the newlyweds leave the chapel.
Following the service, a carriage procession will travel along a route including Castle Hill, High Street, Sheet Street, Kings Road, Albert Road, Long Walk and back to Windsor Castle.
All 600 guests will then attend a lunchtime reception at St George's Hall, which is being given by the Queen.
During this reception, Ms Markle will reportedly break with tradition for royal brides and make a speech.
Later in the evening, the newlyweds will celebrate with 200 close friends and family at a private reception less than a mile from Windsor Castle at Frogmore House, hosted by Prince Charles.
The 17th century country house and gardens - a former home to numerous royals throughout history - was the back-drop for the pair's engagement photos.
The Royal Family will pay for the wedding, including the service, music, flowers and reception.
