Abdullah bin Abdul Aziz was 81 when he rose to the Saudi throne in 2005, but moved like a reformer decades younger to unleash his kingdom from oil dependency and hardline Islamic clerics and propel it into the 21st century.
Tapping deeply into Saudi Arabia's massive oil wealth, Abdullah launched projects to build new economic cities, universities and high speed railways rather than shining palaces and mega-yachts.
At the same time he has tried to convert the kingdom from a breeding ground for Islamic radicals into a moderate, constructive partner in global politics.
But his age and his commitment to consensus have allowed entrenched conservatives to resist his social reforms.
And the lack of clarity about the future path of the monarchy leave many of his changes tentative.
That became more evident in late November when the royal court announced that the king underwent an operation in New York to treat a debilitating herniated disc, which had slowed him for several months.
The royal court announced on Friday that Abdullah would undergo further surgery on his back "to repair several vertebrae in his spine in a follow-up to his previous treatment."
Since the death in 1952 of King Abdul Aziz Al-Saud, the founder of modern Saudi Arabia, the throne has systematically passed from one of his sons to another, brothers and half-brothers.
But all of King Abdul Aziz's sons are now aged. Abdullah's half-brother Sultan, believed to be 84, is crown prince. But even if he succeeds as expected, the day is approaching when the throne must pass to the succeeding generation.
To exactly whom, and how it will take place, remains unsettled.Unstained by the profligacy tainting many of the ruling Al-Sauds, Abdullah is hugely popular with his subjects, cherishing the traditional desert life of the Bedouin.
Behind his thick, always jet-black moustache and goatee and gentle demeanor, he has a shrewd grasp of regional politics.
He was born in 1924, the 13th son of King Abdul Aziz, but the only son of Abdul Aziz by his mother, a member of the Shammar Bedouin tribe.
That left Abdullah with a relatively weak faction among the many princes of his generation.
In the 1960s he was entrusted with the command of the national guard, the country's second army. He held that job until turning it over to one of his own sons in November this year.
The job allowed him to build close relations with the kingdom's myriad tribes who filled the guard's ranks, one of the main pillars of his authority today.
Abdullah became crown prince when his half-brother Fahd ascended the throne in 1982.
Yet his path was still not clear when Fahd was incapacitated by a stroke in 1995. He faced rivalry from Fahd's own Sudairy clan, which included the defense minister Prince Sultan and the interior minister Prince Nayef — all sons of Abdul Aziz by another of his wives.
But since the late 1990s, Abdullah fostered important changes. He developed the consultative Shura council, strengthened the country's finances and began modernizing the unwieldy sharia-based legal system.He took Saudi Arabia into the G20 group of leading economies and the World Trade Organization.
And he has challenged conservatives by supporting progressive clerics, creating human rights organizations and launching a science university that, for the first time, permits men and women students to mix freely.
Nevertheless, his kingdom remains strongly criticized for a dismal human rights record.
Abdullah also led the country's grudging response to Islamic extremism after the September 11 attacks on the United States, which bared Al-Qaeda's deep roots inside Saudi Arabia.
Although he expressed opposition, he permitted the US military to use Saudi facilities and bases for the US invasion of Iraq, but only for logistical support.
In 2002 he parented the path breaking Arab Peace Initiative, which offered Israel blanket recognition from 22 Arab states in return for an independent state for the Palestinians.
In public he has been a committed supporter of diplomacy to solve conflicts, calling for a peaceful solution Iran's nuclear threat.
But US State Department documents disclosed by WikiLeaks this week depicted him privately calling for a US military attack on Iran's nuclear facilities to "cut off the head of the snake," and revealed US suspicions that Al Qaeda received much of its funding from Saudi Arabian sources.