The Queen and the President of Ireland stood side by side to observe the Irish Anthem being played by a military band in Windsor, during the first visit to the UK by an Irish head of state.
The anthem was played after the President Higgins and Michael Higgins were honoured with two separate gun salutes, and Mr Higgins and Philip inspected troops of the Queen's Company, Genadier Guards.
Major Andrew Seddon, captain of the Queen's Company Grenadier Guards, invited Mr Higgins to inspect the guard of honour, speaking to him in Irish. The Irish Guards was represented by their regimental band as the troops are currently on peacekeeping duties in Cyprus.
President Higgins presented a new ceremonial red coat to its regimental mascot, an Irish wolfhound called Domhnall of Shantamon.
The Irish flag is hanging outside Windsor Castle as the Queen prepares to welcome the Irish President for the first state visit in history.
Crowds have started to gather along the route, ahead of Micheal D Higgins' momentous meeting with the Queen.
Although President Higgins has travelled to events in London, Manchester, Liverpool and Scotland last year, these were not official visits.
Today's ceremonial visit will begin at the Irish Embassy, when the Prince of Wales and The Duchess of Cornwall will greet The President and Mrs Higgins, on behalf on the Queen.
They will then journey together to Windsor.
Irish President Michael D Higgins said today's historic state visit to the UK was not about "forgetting the past" or "wiping the slate clean" but about focusing on the present and building future relations. It is the first official state visit from Ireland to its former colonial ruler.
Speaking to Irish state broadcaster RTE he said progress should not be about forgetting the past.
"The challenge is to hand to a future generation all of the prospects of the future. You are not inviting them to an amnesia about any deep dispute.
"There are a lot of very difficult memories and it would be to my mind wrong to suggest to anyone that you should as it were, wipe the slate clean."
The President will be joined on the trip by Irish Taoiseach Enda Kenny and Foreign Affairs Minister Eamon Gilmore.
It's an historic day for the Queen and the Duke of Edinburgh today in Windsor.
They are due to meet the Irish President Michael Higgins.
There will be a traditional carriage procession through the streets to reach the castle.
Video. Windsor Castle has much to entice tourists inside its stone walls but outside in the grounds, there is a new attraction. Six willow arches have been constructed as a gift to the Queen, one for each decade she has been on the throne.
The coronation arches are now on display on the long walk at Windsor Great Park and has taken two months to complete. They were officially opened at the weekend. Divya Kohli reports.
A car that used to belong to Her Majesty the Queen is up for auction in Surrey. The Daimler saloon car was delivered to Buckingham Palace in 2001. The Queen used the vehicle to travel between Windsor Castle and Buckingham Palace, as well as when attending her other engagements.
The car was transferred to The Jaguar Daimler Heritage Trust by the Royal household in 2004, and has a mileage of 15,252. The guide price at Brooklands auctioneers is £25,000 - £30,000.
The car was last seen in public at the Cartier Royal Jubilee display at the Goodwood Festival of Speed in Sussex in 2012.
The description of the car in the guide listing ends: '... this represents a rare opportunity to purchase a unique, important and totally useable, one careful lady owner vehicle which would surely grace any collection.'
Her Majesty was due to attend a military celebration in Wales during the country's St David's Day celebrations, but was forced to cancel her planned engagements.
Buckingham Palace declined to comment on her condition, but a spokeswoman earlier confirmed the Queen would spend the weekend at Windsor.