Here are the timings which have been announced for the day:
9am - Members of the public who have been invited to watch the wedding day from the grounds of Windsor Castle begin to arrive.
9.30-11am - Wedding guests arrive at the Castle's famous Round Tower by coach and enter the chapel through the South Door and take their seats.
11.20am - Members of the Royal Family begin to arrive and enter via the Galilee Porch, some on foot and others by car.
MORE - click here for all our Royal wedding coverage
11.45am - Possibly around this time Harry and his brother the Duke of Cambridge arrive at the Chapel's West Steps, probably on foot walking past the thousands of spectators invited into the grounds of the castle. The moment will give Harry the chance to acknowledge the 200 representatives from charities he is associated with gathered in the Horseshoe Cloister at the bottom of the steps.
11.55 - 11.59am - Following protocol, the Queen will be the last member of the royal family to arrive for the wedding service.
11.59am - Ms Markle arrives at the Chapel's West Steps by car, from her overnight accommodation, with some of her bridesmaids and pageboys. She will start her journey, which will take her past some of the crowds in the Long Walk, with her mother Doria Ragland - but when the car stops at the castle her mother will exchange places with her bridesmaids and pageboys. Ms Ragland will enter the chapel by the Galilee Porch. She was due to be walked down the aisle by her father, Thomas, but he suffered heart problems earlier in the week and was not well enough to fly to Britain from Mexico to attend the wedding.
12.00pm - Service starts.
1pm - Service ends and the newlyweds emerge at the West Steps of the Chapel. They will be waved off on their carriage procession through Windsor by members of both families. The congregation will file out of the chapel to see the newlyweds leave before heading to St George's Hall for the lunchtime reception hosted by the Queen.
1.05pm - Possibly around this time the carriage procession through the streets of Windsor begins, taking 25 minutes.
7pm - Bride and groom depart Windsor Castle for the evening reception at Frogmore House, hosted by the Prince of Wales.
Best man
The Duke of Cambridge will be best man for his younger brother Harry.
He will deliver a speech at the reception and be in charge of the rings. William, president of the Football Association, is missing the FA Cup final in favour of the celebrations.
The night before
Ms Markle and Harry are keeping to tradition by spending the night before their wedding apart.
Dress
The design and designer will be kept secret - even from Harry - until the moment Ms Markle emerges in her gown on the way to the chapel.
Grounds of Windsor
Some 1,200 specially selected members of the public - including charity and community heroes - have been invited into the grounds of the castle from 9am to picnic and watch the royal couple arrive, and leave for their carriage procession.
Timings
The congregation will start arriving from 9.30am, and the royals from 11.20am. The service begins at 12 noon and the newlyweds will emerge from the chapel at 1pm.
Harry and William's arrival
The brothers will arrive at the West Steps, walking past the hundreds of spectators invited into the Lower Ward, as well as greeting representatives from Harry's charities.
Mother of the Bride
Doria Ragland will travel by car with her daughter to Windsor Castle on the morning of the wedding, before exchanging places with the bridesmaids and pageboys and entering the chapel through the Galilee Porch.
Father of the bride
Thomas Markle is reportedly suffering heart problems and will be unable to fly to the UK to walk his daughter down the aisle.
Flowers
The chapel will be filled with white garden roses, peonies and foxgloves, with branches of beech, birch and hornbeam, carefully crafted by floral designer Philippa Craddock.
Guests
Kensington Palace is not releasing an official guest list. Some 600 guests will be in St George's and 200 of those will party into the night at the reception in nearby Frogmore House.
No politicians
Neither Donald Trump, Barack Obama nor Canadian PM Justin Trudeau will be there and neither will Prime Minister Theresa May as aides have said the wedding is not a state occasion.
The Spencers
The siblings of Harry's late mother Diana, Princess of Wales will be guests - Earl Spencer, Lady Sarah McCorquodale, and Lady Jane Fellowes. Lady Jane will give a reading.
Music
Cellist Sheku Kanneh-Mason, the gospel Kingdom Choir founded and directed by Karen Gibson, Baroque specialist trumpeter David Blackadder and soprano Elin Manahan Thomas will be among those providing the music which will echo around the 15th century church.
Choir
The Choir of St George's Chapel was founded in 1348 and is made up of 23 choristers from St George's School, and 12 lay clerks singing alto, tenor and bass. Lay clerks - adult professional singers - live with their families in the Horseshoe Cloister, west of the Chapel.
Kiss
The bride and groom's appearance on the West Steps, surrounded by their families, will be their 'balcony moment' - the most likely opportunity for a public kiss.
Carriage ride
Just after 1pm, the couple will be waved off by members of the royal family and head on a 25-minute carriage ride through the town in the Ascot Landau, with military representatives lining the streets.
Lunchtime reception
This will be in the magnificent St George's Hall at Windsor Castle.
The Queen is hosting the gathering for the 600-strong congregation.
Cake
The wedding cake is a lemon elderflower wedding cake decorated with buttercream icing and fresh flowers. It is being made by Claire Ptak, owner of the trendy, organic Violet Bakery in Hackney, east London.
Photographer
Fashion photographer Alexi Lubomirski, who took the couple's engagement pictures, has been chosen to capture the big day as the official wedding photographer.
Royal baby
Prince Louis will not be there, but his mother the Duchess of Cambridge will be attending.
Duke of Edinburgh
The palace is hoping that 96-year-old Philip, who is recovering from a hip operation, can make it to the ceremony, but his presence is not yet confirmed.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here