Martin McGuinness is stepping down as Deputy First Minister of Northern Ireland in protest against the Democratic Unionist Party's handling of the botched Renewable Heat Incentive (RHI) scheme - which has left Stormont with a £490 million bill.
Here's everything you need to know about the man whose move is likely to shake the DUP/Sinn Fein administration, possibly leading to a snap Assembly election.
What do we know about McGuinness's IRA past?
McGuinness has always acknowledged his IRA past. In 1972, at the age of 21, he was second-in-command of the IRA in Derry, a position he held at the time of Bloody Sunday, when 14 civil rights protesters were killed in the city by soldiers with the 1st Battalion, Parachute Regiment.
The following year he was convicted by the Republic of Ireland's Special Criminal Court after being arrested near a car containing explosives and ammunition.
After his release, and another conviction in the Republic for IRA membership, he became increasingly prominent in Sinn Fein, eventually becoming its best known face after Gerry Adams.
He was in indirect contact with British intelligence during the hunger strikes in the early 1980s, and again in the early 1990s.
How did me make the move to politics?
In 1982, McGuinness was elected to the Northern Ireland Assembly at Stormont representing his home city of Derry. He was the second candidate elected after John Hume.
But as with all elected members of Sinn Fein and the SDLP, he did not take his seat.
He was elected to the Northern Ireland Forum in 1996 representing Foyle. Having contested Foyle unsuccessfully at the 1983, 1987 and 1992 Westminster elections, he became MP for Mid Ulster in 1997.
When did he become the deputy leader at Stormont?
McGuinness became Deputy First Minister of Northern Ireland on May 8 2007, with Democratic Unionist Party (DUP) leader Ian Paisley becoming First Minister.
In June 2008 he was reappointed as Deputy First Minister to serve alongside Peter Robinson, who succeeded Paisley as First Minister.
His unlikely journey took him from being second-in-command of the Provisionals in Londonderry on Bloody Sunday to Deputy First Minister (DFM) at Stormont.
During his time as DFM he forged such a good working relationship with Paisley that they were dubbed "the Chuckle Brothers".
But it was his more strained relationships with Paisley's successors, Robinson and Arlene Foster, that led to continual difficulties in recent years at the top of the power-sharing executive.
What are his contributions?
McGuinness left his IRA past behind to become one of the major players in Northern Ireland's peace process and later became Sinn Fein's chief negotiator in the negotiations that led to the Good Friday Agreement - a peace deal that brought an end to conflict in the state.
The deal was made between the British and Irish governments and political parties in Northern Ireland about how Northern Ireland should be governed.
However the agreement proved difficult to implement and was amended by the St Andrew's Agreement in 2006. The deal brought the DUP and Sinn Fein - once staunch enemies - together and McGuinness and Paisley emerged as Northern Ireland's new political leaders.
During his time as minister he scrapped the 11-plus exam, which he had failed as a schoolboy.
How has Foster reacted to his decision to step down?
First Minister Arlene Foster has accused McGuinness of putting politics before principle by tendering his resignation.
Foster said: "It is clear that Sinn Fein's actions are not principled, they are political.
"His actions have meant that, at precisely the time we need our Government to be active, we will have no government and no way to resolve the RHI problems."
This comes after McGuinness repeatedly insisted that an independent investigation into the botched RHI scheme cannot go ahead unless Foster steps aside.
The scheme - which has left Stormont facing a £490 million overspend - was set up to encourage uptake of renewable heat technologies but failed because the subsidy tariffs were set too high.