DUP leader Ian Paisley and Sinn Fein's Martin McGuinness are to take their pledge of office as devolution returns to Northern Ireland.
Ian Paisley and Martin McGuinness will take their pledges
British and Irish Prime Ministers Tony Blair and Bertie Ahern are at Stormont to witness the ceremony.
Direct rule over NI by London ministers officially ended at midnight, almost five years since it was brought back.
As he arrived at Stormont, Mr Paisley said Northern Ireland was "on the road to prosperity".
"I believe we're starting on a road which will bring us back to peace and to prosperity," he said.
"And I would challenge the people of Northern Ireland to rise to the challenge and be determined that, come what may, we'll make this a country where all men and women will be equal under the law - and equally subject to the law."
Mr McGuinness said he was confident he and the DUP leader could work together.
"We've already taken joint decisions, but that was in the context of not having power," he said.
"All of that is going to change in the next couple of hours, and by midday today, we're going to be in charge, and we're going to be charged with the responsibility of governing in the interests of the people."
Sinn Fein president Gerry Adams said it was "a big day" for the people of Ireland, and that his party and the DUP had opened up "the potential for a new beginning".
"I think what today proves is that dialogue and perseverance and tenacity and persistence can bring about results," he said.
Both Mr McGuinness and Mr Paisley paid tribute to DUP assembly member George Dawson, who died on Monday evening.
It is expected proceedings will be suspended for 30 minutes as a mark of respect for the East Antrim representative.
Ministers from the four main parties will take their pledge of office, which includes support for the police.
The return of devolved government follows an historic meeting in March between DUP leader Ian Paisley and Sinn Fein president Gerry Adams, where they agreed to share power.
Northern Ireland Secretary Peter Hain said he was confident the parties would make a go of it.
"It's going to stick, I believe, because the DUP and Sinn Fein - Gerry Adams and Martin McGuinness on the one hand, Ian Paisley and Peter Robinson on the other - these are the two most polarised forces in Northern Ireland's politics, they have done the deal," he said.
In October 2002, allegations of intelligence gathering within Stormont led to the suspension of power-sharing institutions. A subsequent court case collapsed.
Among the VIP guests on Tuesday will be US senator Ted Kennedy.
The DUP leader's wife Baroness Paisley and the Sinn Fein chief negotiator's mother Peggy McGuinness are also expected to attend.
A new speaker, the DUP's William Hay, is to be elected assembly speaker along with three new deputies from the other main parties.
The first meeting of the new power-sharing executive is scheduled for later this week.