Irish President Michael D Higgins has been welcomed to the UK by the Royal Family on the first state visit by an Irish head of state.
It is another milestone in the thaw in relations between the two countries following decades of tension over Northern Ireland.
The president was greeted by Queen Elizabeth at Windsor Castle. It comes three years after a visit by the Queen to the Irish Republic, the first by a British monarch since Dublin gained independence from London in 1921.
Northern Ireland remained under British control – and from the 1960s to the 1990s several thousand people died in the conflict between mainly Catholic Republicans seeking unification with the south, and Protestant Unionists wanting to stay in the UK.
In a further sign of reconciliation, Irish Republicans from Sinn Fein are among the delegation.
They include Martin McGuinness, a former IRA commander.
Its bombing campaign against British rule claimed many civilian lives and his presence has angered some families of IRA victims.