By Gordon Corera
BBC security correspondent
The kidnapping of a Briton, two Canadians and an American in Baghdad on Saturday is a reminder that Iraq remains a dangerous place for foreign nationals.
Ken Bigley's case was among the most high-profile kidnappings
Even though there have been fewer high-profile kidnappings in recent months, the problem has not gone away.
A total of 238 foreign nationals were kidnapped in Iraq between May 2003 and November 20th, 2005, according to the Brookings Institution's Iraq Index.
About one in five were killed. By some estimates as many as 5,000 Iraqis were kidnapped over a shorter period of only 17 months.
The peak period for kidnapping foreign nationals came from April to September 2004 when a total of 135 were taken, although it was later in the autumn that two of the most high-profile cases occurred - those of Ken Bigley and Margaret Hassan.
In many of the cases, it is thought that criminal gangs initially took foreign nationals hostage.
In some cases they asked for a ransom themselves, at other times they sold them onto militant groups with political aims.
Some groups hold the hostages as a bargaining chip - Shia groups like that of Moqtada Sadr are thought to have taken hostages to secure the release of some of their organisation's leaders who were being detained by coalition forces.
Guardian journalist Rory Carroll was held by a Shia group for 36 hours before being released. Many of these groups are susceptible to forms of influence.
The most dangerous group is that of Abu Musab al-Zarqawi - here the intent is to use the captive to spread fear. Zarqawi began the trend of putting out videos, including some showing the executions of those he had held.
Since last summer, there has been a decline in the numbers kidnapped.
A letter by Ayman al-Zawahiri may have led to a change in tactics
It is not clear how much of this is due to a change in tactics by insurgents and how much is due to the publicity of last summer leading to many organisations either pulling foreign nationals out of Iraq or increasing their security to make it harder for potential kidnappers.
There has been speculation that a letter from al-Qaeda's number two, Ayman al-Zawahiri, possibly to one of Abu Musab al-Zarqawi's aides might have led to a shift in tactics.
In the letter (whose authenticity has been the subject of debate), Zawahiri appears to warn Zarqawi's group that his violent tactics of slaughter are alienating moderate Muslims and proving counter-productive.
However there is no sign of Zarqawi heeding any warning, as witnessed by his attack on the hotels in Jordan on 9 November.
More recently, Zarqawi has also been busy kidnapping Arab diplomats - including Moroccans, Algerians and Egyptians - and killing them as a message to their countries not to recognise the new government of Iraq.
As has happened in Afghanistan, it is also now clear that no group or nationality is immune from attack.
Previously, nationals of some countries felt they were immune from being taken because their country opposed the war.
But the kidnapping of two French journalists proved that wrong (they were later released).
Aid workers like Margaret Hassan who had deep roots in the country and in helping the people of Iraq were also shown not to be immune.
As has been the case with the insurgency as a whole, the tactics of kidnapping hostages look to have evolved over time, trying to establish the best way of getting round new security measures and of maximising impact.