President Michael D Higgins of Ireland has been welcomed to the United Kingdom by the Queen and other members of the Royal Family at Windsor Castle.
It's the first visit by an Irish head of state since independence in 1922.
The ceremonial welcome at Windsor Castle was marked by a 21-gun salute, military bands and marching troops.
Mr Higgins then visited Westminster Abbey and laid a wreath at the Grave of the Unknown Warrior - the tomb of a British soldier of World War One. This is customary on state visits.
The BBC reports he and his wife Sabina also stopped to look at a memorial to the Queen's cousin, Earl Mountbatten of Burma, who was killed by an IRA bomb in 1979.
In a speech to both Houses of Parliament in the Palace of Westminster, Mr Higgins said: "I stand here at a time when the relationship between our two islands has, as I have said, achieved a closeness and warmth that once seemed unachievable."
He spoke of the "pain and sacrifice" associated with Irish independence from the UK in 1922, which he said had cast a "long shadow across our relations".
"We acknowledge that past but, even more, we wholeheartedly welcome the considerable achievement of today's reality - the mutual respect, friendship and co-operation which exists between our two countries," he said.
The Queen visited the Republic of Ireland three years ago - the first British monarch to do so.
Ireland won independence following a civil war, but six counties were kept under British control, creating Northern Ireland.
President Higgins said before his visit that there were "a lot of very difficult memories" and that it would be wrong to "wipe the slate clean".