Jonathan Evans has become the new head of the security service MI5 after being the deputy for two years.
Mr Evans has been in the security service for 27 years
Mr Evans, 49, took over the head position following the retirement of Dame Eliza Manningham-Buller.
He has been in the security service since 1980, specialising in counter-terrorism - both domestic and international.
As head of MI5, Mr Evans will be responsible for leading the battle against terrorism within the UK.
Dame Eliza warned last year that the UK faced a "serious" and "growing" threat from terrorism.
Mr Evans, who is the service's 16th director general, spearheaded the fight against Irish republican terrorism in the late 1980s and 1990s.
He then served as the head of international counter-terrorism investigations from 1999 to 2001.
In 2001 he was appointed to the security service's management board as director of international counter terrorism - 10 days before the 9/11 attacks in the US.
MI5 says since then Mr Evans has "played a leading role in tackling the threat of al-Qaeda and related terrorist networks."
MI5 increased in size by nearly 50% since the September 11 attacks, meaning Mr Evans will head a staff of around 2,800.
Mr Evans graduated from Bristol University with a degree in Classical Studies.
On joining the security service in 1980 he worked on counter-espionage investigations for five years.
During his career, he also helped implement policy changes as part of Sir Anthony Duff's modernisation of the service during the 1980s.
And during a two-year attachment to the Home Office he was involved in the development and implementation of VIP security policy.