The Queen's love of horses is well documented and that passion is reflected in the celebrations over the next few nights.
As a young princess she enjoyed riding in Windsor Great Park - something she still does today.
The Queen owns and breeds thoroughbreds. Those close to her say horse racing is when she really relaxes. Penny Silvester has been talking to the Queen's Racing Advisor, John Warren, about why her affection for these animals runs so deep.
On Sunday, The King's Troop will perform in front of the Queen and it's taken months of training to get them ready.
The soldiers on horseback were first formed in 1946 to be a show piece during ceremonies of state. Simon Parkin went to see them rehearsing.
The Queen's 90th birthday celebrations in Windsor this weekend are set to be an event to remember. The offering will feature one and a half thousand performers from across the world, 900 horses, 25,000 spectators and a very special guest of honour.
We've met some of the acts taking part and we've been speaking to those who know the Queen personally.
Last-minute sound checks and rehearsals have been happening today in the main arena, ready for this evening's events. But the day started with the 73rd Royal Windsor Horse Show. Here's Mike Pearse.
The American President swept into the south today for a special birthday lunch with the Queen.
Barack Obama, accompanied by the first lady - arrived by helicopter at Windsor Castle. They were greeted by her majesty and the Duke of Edinburgh who were waiting in the Berkshire rain.
But Mr Obama also flew straight into a political storm as he used his last official visit as president to urge the UK to stay within the EU.
With more from Windsor, here's Divya Kohli.
Her Majesty The Queen is ninety years old today, and Windsor in Berkshire is the focus of the celebrations. Queen Elizabeth II will go on a walkabout around the town, accompanied by her husband, the Duke of Edinburgh.
The Queen will also unveil a plaque marking 'The Queen's Walkway', a 6.3km self-guided walking trail connecting 63 points of significance in the town of Windsor. The Walkway symbolises the moment at which Her Majesty became Britain's longest reigning monarch, after more than 63 years on the 9th September 2015.
Celebrations will also take place in June to mark Her Majesty The Queen's official birthday.
Thousands of people have been applying for free tickets to help celebrate the Queen's birthday in Windsor.
Tickets for the celebration on the Long Walk in May sold out within hours in November. But a ballot of 5,000 tickets opened to the public yesterday.
The Long Walk will offer the opportunity to be involved in a pre-performance party on the final night of the event and to watch a live-screening of the ITV broadcast from the iconic Long Walk.
The show will tell the remarkable story of The Queen's life encompassing a reign spanning more than 60 years, starting from when she was born in 1926, through World War Two, to The Coronation in 1953.
Applications for the ballot can be completed online by filling out a short form, which can be found at www.hmq90.com or they can be applied for in writing to: Celebration on The Long Walk, The Old Booking Hall, Windsor Royal Shopping, Thames St, Windsor SL4 1PJ.
It is being billed as a huge party for the Queen - a star studded show in the grounds of Windsor Castle, to mark her Majesty's 90th birthday.
Now it has been revealed the event in May, which will be broadcast live on ITV, will be hosted by Britain's most popular TV duo Ant and Dec. As Rachel Hepworth reports, preparations are well under way.
The interviewees are: Simon Brooks-Ward, Director of the Queens's birthday celebrations; the singer Katherine Jenkins; Pau Roach, Windsor & Eton town manager; and Julia White, Windsor Visitor manager.