Peter Robinson and Martin McGuinness have been appointed first and deputy first ministers of the Northern Ireland Assembly.
Mr Robinson was nominated by former DUP leader Ian Paisley and Mr McGuinness by Sinn Féin president Gerry Adams.
The new first minister said it was important that "people learned from the past, not live in it".
The deputy first minister said the "honeymoon period" at Stormont was over and that it was time to deliver.
Addressing a special sitting of the assembly, Mr Robinson said: "If devolution is to be meaningful to the people of Northern Ireland then the responsibility for delivering results rests with all of us.
"Of course the deputy first minister and I will have to roll up our sleeves - and neither of us will duck the challenges or recoil from the hard work that such tasks require."
Mr McGuinness said: "The honeymoon period is over. This is now about hard work, people out there expecting results and we are here to deliver."
He added: "We are determined to bring an early conclusion to the talks that begin tomorrow to resolve all of the issues outstanding from the St Andrews Agreement."
I think this 12 months that we've had has been one of the most important 12 months of Ulster's modern history
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