Michael Higgins has become the first Irish President to make a state visit to the UK, with a warm welcome laid on today by the royal family and parliamentarians.
He stressed ahead of the trip that this was not about "wiping the slate clean" or forgetting the difficulties of the past, but about celebrating a mature, friendly relationship between Ireland and its former colonial ruler.
President Higgins began his day at the Irish embassy, where Prince Charles and the Duchess of Cornwall greeted him before the party travelled to Windsor Castle, where an Irish flag fluttered alongside a Union Jack.
There was also a small protest outside the castle from relatives of the victims of IRA bombings angry that Sinn Fein's Deputy First Minister, Martin McGuinness, was invited to the state banquet this evening.
Victor Barker, whose son was killed in the Omagh bombing in 1997, said he wanted people to be "reminded of McGuinness' past" as a member of the IRA.
Inside the castle, the Queen and President stood side by side for the Irish national anthem, before being introduced to members of the Irish Guards regiment.
In the afternoon President Higgins travelled to Westminster for a historic address to both Houses of Parliament, in which he saluted the "warm friendship" between the UK and Ireland and praised the contribution of Irish immigrants to British life.
The Queen offered similar sentiments at a state banquet this evening, saying:
"We, who inhabit these islands, should live together as neighbours and friends. Respectful of each other's nationhood, sovereignty and traditions."
The warmth of the Anglo-Irish relationship was also demonstrated when Historical Royal Palaces bathed the Tower of London in green light in tribute to the state visit.