A silver-haired university president, Gates is the polar opposite of the fiery Donald Rumsfeld, who is stepping down after nearly six years in office.
Cool-tempered, bright and boyish-looking despite his 63 years, Gates speaks with the clipped, concise diction of the intelligence analyst he was for nearly 27 years at the Central Intelligence Agency.
"He's a man of integrity, candour and sound judgement," Bush said at the White House, flanked by Gates and Rumsfeld.
"He knows that the challenge of protecting our country is larger than any political party. And he has a record of working with leaders on both sides of the aisle to strengthen our national security," he said.
In naming Gates as Rumfeld's successor, Bush appeared to be reaching back to a group of more pragmatic advisers who surrounded his father during the Gulf War when the United States drove Iraqi forces out of Kuwait but did not follow them to Baghdad.
Senator Joseph Biden, a Democrat, said his sense was that Gates had "a much more pragmatic and realistic view of the place we find ourselves. And the first thing I'm looking for is pragmatism."
An expert on the United States's Cold War adversaries, Gates inherits a military mission in Iraq that is entangled in a deepening sectarian conflict and another in Afghanistan that has heated up again over the past year.
He is a member of the bipartisan Iraq Study Group headed by former secretary of state James Baker, another veteran of the elder Bush's administration, and Democrat Lee Hamilton.
The group has been working up a set of recommendations that commentators have predicted could form the basis of a new bipartisan approach to Iraq.
With the Republican Party's loss of control of the Congress in Tuesday's stunning defeat in midterm elections, Gates will be key to bringing disaffected Democrats on board.
Despite wide consensus that the Iraq occupation is in deep trouble, there is little agreement on what to do about it.
Options include setting a timetable for the withdrawal of US forces, the division of the country into three autonomous regions, the deployment of more US troops to reduce the violence, and an open-ended occupation that banks on building up Iraqi forces to take over from US troops.
Where Gates stands was not know immediately known.
President George Herbert Walker Bush, the current president's father, tapped Gates to be his director of the CIA in 1991 through January 1993.
Gates had served as the CIA's deputy director, the foil to President Ronald Reagan's legendary spymaster William Casey, another impetuous personality at the center of the Iran-Contra scandal.
He later served as deputy national security adviser under the elder Bush from 1989 to 1991.
Since leaving the CIA, Gates has served as president of Texas A and M University in College Station, Texas.
Bush described him as "a steady, solid leader who can help make the necessary adjustments in our approach to meet our current challenges."
"If confirmed by the Senate, Bob will bring more than 25 years of national security experience and a stellar reputation as an effective leader with sound judgement," he said, noting that he "rose from an entry-level employee in the CIA to become the director of central intelligence."
During his service Gates "gained first-hand knowledge that will help him meet the challenges our country faces during the next two years."

Copyright AFP (Agence France-Presse), 2006