With growing concerns about the terrorist threat to Britain, BBC News Online looks at the different organisations tasked with the job of defending the UK.
Officially known as the Secret Intelligence Service (SIS), its role is exclusively involved in obtaining intelligence on the activities of Britain's enemies and potential enemies overseas.
Up until a decade ago the main threat came from the Soviet Union and other Warsaw Pact countries.
Following the collapse of the Soviet Union MI6 shifted its main focus to Irish republican terror groups.
But since 11 September 2001 MI6's primary function has been to keep an eye on the threat from Muslim extremists.
MI6 will have agents in, for example, Afghanistan, Iraq, Yemen, who are designed to give early warning of potential threats to Britain from al-Qaeda.
MI6 is an arm of the Foreign Office and its boss, known as C, is answerable to the foreign secretary.
It used to be said that MI6 agents worked abroad and MI5 - known officially as the Security Service - worked within the UK's borders.
But nowadays that is not the case. MI5 agents often travel abroad to liaise with foreign intelligence agencies and share information on those who might threaten British national security.
MI5 is also concerned with threats to the security of British embassies and consulates abroad and British expats living overseas.
It operates a network of agents and also works closely in support of Special Branch, which has its own informants in a variety of groups which are of concern to the government.
These include Irish republicans, Northern Irish loyalists, Kurdish separatists and Muslim extremists.
These informants - known in the trade as Covert Human Intelligence Sources - have defected for a variety of reasons.
Usually it is because they are disillusioned by their movements' actions, but it can also be because of financial inducements offered to them.
MI5 is funded through the Single Intelligence Account, which also supplies the budget for MI6 and GCHQ.
Its boss is answerable to the home secretary.
Whereas MI5 and MI6 are both classed as the intelligence service, they work closely with Special Branch, whose officers are policemen and women.
Each of the UK's police forces has a Special Branch, whose officers are given the task of keeping tabs on suspect organisations in their area.
Special Branch officers also keep surveillance - both physically and technologically - on individuals who are considered "of interest" to the powers-that-be.
Telephone are tapped, e-mails monitored, mail intercepted and homes bugged in an attempt to obtain valuable intelligence on the people under surveillance.
Special Branch, and MI5, are supposed to tip off the local police if they discover a plot is imminent.
The Metropolitan Police, being the biggest and best-funded force in the country, is the only constabulary in the UK to have a dedicated Anti-Terrorist Squad.
Anti-Terrorist Branch officers are often sent miles from London in the course of their work and are supposed to work closely with local officers if they discover terrorist activity in, for example, Lancashire.
In December 2001 Scotland Yard officers boarded a cargo ship in the English Channel because of fears that it might be carrying terrorist material. In the end the ship was given the all-clear.
The Anti-Terrorist Branch has years of experience in tackling terrorism, most noticeably that of Irish republicans operating on the British mainland.
They have faced a steep learning curve with Islamic extremists since 11 September 2001 but the lack of a major terrorist incident in Britain suggests they are getting results.
NATIONAL CRIMINAL INTELLIGENCE SERVICE
NCIS was established in 1992 and provides intelligence on major organised crime gangs for police forces up and down the country.
But it will, from time to time, in the course of its activities come across terrorist conspiracies.
Several terror groups, such as the Ulster Volunteer Force (UVF), the Ulster Defence Association (UDA), the Kurdish separatist PKK and the Turkish left-wingers DHKP-C, fund their activities through extortion and drug dealing.
There is therefore considerable overlap and NCIS will frequently provide information to MI5 or the Anti-Terrorist Branch.
While the above groups all work with intelligence from human sources, GCHQ is responsible for monitoring the airwaves.
GCHQ - which stands for Government Communications Headquarters - was set up during World War II to decode German coded messages.
Now based in Cheltenham, Gloucestershire, its staff monitor a wide range of communications globally.
Equipped with a vast array of state-of-the-art technology - work began on a £300m facility in 1999 - they are able to listen in to communications between virtually anyone in the world.
Their work, obviously, is shrouded in secrecy but GCHQ will be at the forefront of efforts to locate secret messages between Osama Bin Laden and his minions.