Muslim extremists from Britain and other countries have travelled to the camps to receive instructions on how to make bombs, shoot guns, kidnap and torture and plot atrocities against Westerners.
Most of the al-Qaeda terrorist training camps in Afghanistan were destroyed in the international military action that followed the September 11 attacks in the United States.
However, terrorist training continued in remote regions of Pakistan and Kashmir. Many of those trained by al-Qaeda returned to their home countries, including Britain, where they joined existing networks or established new, loosely-knit groups.
Almost every Briton suspected of terror activity since the September 11 attacks have received instruction from militants linked to Kashmiri separatists or al-Qaeda.
Counter-terrorism officials and experts said the scale, sophistication and targets involved in the Mumbai attacks were markedly different from previous terrorist plots in India and suggested the gunmen had received training from outside the country.
Roger W Cressey, a former White House counter-terrorism official in the Clinton and Bush administrations, said of last week's attacks in Mumbai: "What is striking about this is a fair amount of planning had to go into this type of attack. This is not a seat-of-the-pants operation. This group had to receive some training or support from professionals in the terrorism business."
MI5 believes the number of potential terrorists living in Britain could be as high as 4,000. Most will have Pakistani links through their families but will have been born and brought up in Britain. The terrorists are usually radicalised through their contact with extremist clerics in British mosques.
This indoctrination is further bolstered through internet contact with other jihadis. Their message is that Islam is under global attack and that simple measures such as fundraising are not enough and that they must prepare for a jihad. They then make trips, sometimes several times, to meet Mujahideen fighters in terrorist training camps in Pakistan and Afghanistan.
Intelligence officials are deeply concerned about the trend toward terrorist groups recruiting Westerners for training in terror camps.
Dhiren Barot was one of the first to make the journey to the region in 1995. Barot, who lived in England, converted to Islam aged 20 and travelled to Pakistan and Kashmir, where he was taught how to use guns and explosives. He wrote a book about his experiences, in which he claimed that British Muslims were better off attacking their homeland than taking part in foreign conflicts.
At least three of the bombers in the July 7 London attacks are thought to have followed in his footsteps. Mohammed Sidique Khan, the mastermind of the attacks, grew up in Beeston, near Leeds, and is thought to have travelled several times to such camps.
In 2007 Mohammed Junaid Babar, the al-Qaeda supergrass, gave a wealth of detail about a camp in Pakistan where there was training in bomb making and weapons.
Just last week a student from London, Mohammed Abushamma, admitted trying to get to Afghanistan in order to join militants fighting against coalition forces.
Some of the militants have claimed they only wanted to fight in Kashmir or Afghanistan or to learn how to defend fellow Muslims in other war zones, such as Bosnia or Chechnya. But they have often gone on to plot atrocities in the UK or to attack Westerners or tourists in other countries.
Terrorism in Pakistan is mainly a result of Pakistan's support of terrorist activities in its neighbouring countries, namely India and Afghanistan, through state funding of Islamic terrorists.
The amount of training and the number of camps in the tribal areas of Pakistan is difficult, if not impossible, to estimate. The camps are small, often sited on remote mountaintops. They are easy to move, and they can be difficult to track by satellite because of the region's rough terrain. One estimate recently put the number at about 52, in Pakistan and Pakistan-occupied Kashmir, but there could be fewer or far more.
Lashkar-e-Taiba is said to have training camps spread across Pakistan and Pakistan occupied Kashmir (PoK). Its camps and recruitment centres are thought to be spread across the length and breadth of Pakistan and PoK including Muzaffarabad, Lahore, Peshawar, Islamabad, Rawalpindi and Karachi.
According to the South Asia Terrorism Portal, which has an extensive database of terror groups and their activities, new recruits to Lashkar-e-Taiba go through a fresher course, and can then proceed to two months' training in the handling of AK series rifles, pistols, rocket launchers and hand grenades. It also provides a 21-day training programme and a three-months specialised programme.
Other methods of training known to take place at terror camps linked to al-Qaeda include map reading, military tactics, torture methods, communication methods, personal security, kidnapping and espionage. Thousands of people are estimated to have passed through the camps. Although some have been caught or killed, many remain at large.
In the past few years videos released from the camps have shown men training with machine guns and heavy weapons. They are shown practising daring manoeuvres, firing their weapons as they hang from ropes and run through obstacles.
Last year militants released a 46-minute videotape depicting some 250 graduates of a Taliban training camp near the Afghan-Pakistan border, which included speeches in English by recruits who were grouped by the countries they had been trained to attack, including Germany and the United States.
A growing force
Lashkar-e-Taiba (LeT), or Army of the Righteous, is a group of Pakistani militants linked to al-Qaeda. Set up in 1993, its primary objective is to end Indian rule in the disputed region of Kashmir.
LeT is the military wing of the Markaz-ud-Dawa-wal-Irshad, an Islamic fundamentalist organisation in Pakistan, headed by Prof Hafiz Muhammad Saeed, who is also the amir, or general, of LeT.
The group has long fought Indian forces in Kashmir and was blamed for an attack on India's parliament in December 2001.
Though banned in 2002 by the Pakistani government, it is reported to have received clandestine support from army and intelligence officers.
Pakistan denies providing any support to LeT.