Sinn Fein veteran Martin McGuinness has been deputy First Minister of Northern Ireland since 2007.
Over the course of his political career he has transitioned from a former IRA commander to being the face of Sinn Fein in Northern Ireland.
Mr 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.
He was in indirect contact with British intelligence during the hunger strikes in the early 1980s, and again in the early 1990s.
In 1982 he 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.
Mr McGuinness became Sinn Fein's chief negotiator in the negotiations that led to the Good Friday Agreement.
The 66-year-old was a key architect in delivering the peace deal and in recent years made further history as he met the Queen on a number of occasions.
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.
After the Good Friday Agreement was concluded, he was returned as a member of the Assembly for the same constituency, and nominated by his party for a ministerial position in the power-sharing executive where he served as education minister between 1999 and 2002.
His political career has spanned numerous momentous occasions, from the first IRA ceasefire in 1994 to the signing of the Good Friday Agreement in 1998, from the decommissioning of weapons in 2005 to power-sharing with the DUP two years later.
Mr McGuinness served as deputy First Minister with three DUP First Ministers - the late Ian Paisley, Peter Robinson and the current First Minister Arlene Foster.
McGuinness was first elected as deputy First Minister in 2007 alongside the late Ian Paisley, in the wake of the 2006 St Andrews Agreement.
They earned the name the "chuckle brothers" following images of them joking and laughing in each other's company.
In 2008 Ian Paisley was succeeded by Peter Robinson. At this time Martin McGuinness said Stormont's "honeymoon period" was over.
Despite rocky beginnings he also formed a business-like bond with Paisley's successor.
Mr McGuinness stood down temporarily to contest the Irish presidential election in 2011 - a time when his IRA past received a lot of attention in the media - a year after the publication of the 2010 Saville Report into Bloody Sunday.
Mr Robinson and Mr McGuinness oversaw the devolution of policing and justice in 2010, while the Stormont House Agreement of 2014 and the Fresh Start Agreeement in 2015 attempted to address key contentious issues between the DUP and Sinn Fein.
Mr Robinson stood aside as First Minister and DUP leader and was succeeded by Arlene Foster in January 2016.
Despite tensions and being on opposing sides of the recent Brexit vote - the pair had a positive working relationship until the scandal over the Renewable Heat Incentive emerged.
Mr McGuinness said he resigned as deputy First Minister of Northern Ireland over Mrs Foster's refusal to stand aside during an investigation into the botched scheme.
In a statement, the MLA said his resignation was also in protest at what he called the “DUP's failure to accept the principles of power-sharing and parity of esteem”. He also mentioned Nama and Red Sky.
"Today is the right time to call a halt to the DUP’s arrogance,” he said on Monday.
"There will be no return to the status quo.”
Sinn Fein President Gerry Adams said: "Martin McGuinness has led from the front in the Executive for the last ten years, defending the integrity of the political institutions and realising the potential of the Good Friday Agreement."
Speculation had surrounded Mr McGuinness's health after he cancelled a planned trip to China with Mrs Foster last month on medical advice.
Mr McGuinness has said his health has nothing to do his resignation.