Air Force General Michael Hayden has been formally sworn in as the new director of the CIA.
His nomination had its critics but the former director of the National Security Agency is used to having to fend off attacks.
A number of lawmakers, including some from President George W Bush's Republican party, voiced concern about Gen Hayden being a general with close ties to the military and his role in an eavesdropping programme has been criticised by both Democrats and Republicans as a violation of civil rights.
As head of the NSA - the American electronic eavesdropping organisation - the 61-year-old oversaw the programme, which allows for the monitoring of international calls and e-mails of terrorist suspects inside the US without a warrant.
It was ordered by President Bush after the 11 September attacks, but it was Gen Hayden who took the lead in publicly defending the policy after it was disclosed in the New York Times at the end of last year.
But Gen Hayden managed largely to sidestep the eavesdropping controversy in his Senate confirmation hearings, with the Senate backing his appointment by 78 votes to 15.
President Bush ordered the eavesdropping operation
While he has been an energetic defender of President Bush's policies, he has been described by top White House aide Dan Bartlett as a "non-conformist and an independent thinker".
He is said to be widely respected in Congress as a consummate briefer on intelligence matters due to his broad knowledge and professionalism.
General Hayden has served for more than 35 years in the Air Force and many in Washington believe that it is simply wrong to have a military officer as head of the civilian CIA.
Rep Peter Hoekstra, chairman of the House Intelligence Committee, expressed concern that this could enhance Pentagon influence over US intelligence operations.
The US Defence Department controls more than 80% of the intelligence budget, with many of the country's 16 intelligence agencies based at the Pentagon.
"The danger of having the military take over intelligence is that the military has a very different perspective on the world," Mr Hoekstra said. "They're worried about today and wars and threats to the United States in the short term and how we might respond
It is not, however, unprecedented for a military officer to run the CIA.
General Hayden is widely seen as an expert in technological intelligence
gathering. The NSA, which he directed from 1999 to 2005, is reliant on satellites to intercept communications and computers to help break enemy codes.
While he was head of the NSA, he acknowledged that the agency should shoulder some of the blame for the intelligence blunders associated with the 9/11 attacks and the Iraq weapons programme.
Last year, General Hayden became the top deputy to the new national intelligence director, John Negroponte, who oversees the CIA and 15 other intelligence agencies.
He is known for directness - some say this comes from his working-class roots. The son of a welder, he was brought up in Pittsburgh.
He worked part-time as a taxi driver to help fund his bachelor's and master's degrees in history at Duquesne University between 1967 and 1969.
His career in the Air Force led him to serve in senior intelligence jobs in Germany during the civil war in the former Yugoslavia.
Between 1980 and 1982, he was chief of intelligence at the Osan Air Base in South Korea.
As a defence attaché in Bulgaria during the Cold War, according to reports, he would dress up as a workman to eavesdrop on the conversations of Bulgarian conscripts.
He is said to love Shakespeare.