GP Koirala is one of Nepal's most long-standing and experienced politicians and has led the country through some of its biggest upheavals.
GP Koirala's career spans seven decades
First coming to prominence as a union leader in the late 1940s, he has been imprisoned, exiled or detained on numerous occasions for championing the cause of democracy against various autocratic regimes.
In 1960, he was jailed for eight years for his pro-democracy views.
His appointment as prime minister is not the first time he has been elevated to the country's leadership after a king's powers have been curbed.
As general secretary of the Nepali Congress Party, he played a key role in public demonstrations in the early 1990s to end the absolute powers of the current king's brother, Birendra.
In 2001 King Birendra was murdered - along with nine members of his family.
That incident occurred while Mr Koirala was prime minister and it was his role to provide leadership for a stunned nation.
While Birendra was king, he had Mr Koirala placed under house arrest during protests against the king's rule. The same fate befell Mr Koirala in the more recent demonstrations against King Gyanendra.
The eighty-five-year-old describes himself as a centrist and a free marketeer.
He has been prime minister on three previous occasions from the early 1990s. His longest stint in the top job was three and a half years.
On each occasion his administration fell due to a combination of Congress Party in-fighting and the narrowness of his majorities in parliament.
Mr Koirala is reluctantly taking the top job, complaining that his ill-health and old age makes him an unsuitable candidate.
It will be a tough work schedule for the octogenarian, who commutes by a Dakota passenger aeroplane from his home in the south-east of Nepal to Kathmandu.
While he has in the past been criticised for being too authoritarian, his longevity and stature mean that he remains one of the few politicians in Nepal capable of achieving political consensus.