The Islamic State jihadist group, which holds large areas in Iraq and Syria and has affiliates and operatives in other countries, claimed attacks in Paris that killed 129 people.
But how does IS function, and who calls the shots?
-Who's in charge?
The group is headed by Abu Bakr al-Baghdadi, who rules over a self-declared Islamic "caliphate", but military and administrative responsibilities are diffused to lower-level officials as well.
IS has various departments responsible for issues such as education and services, and there are also security commanders for specific geographic regions.
"In the grand scheme of things Baghdadi seems to be an important figure in decision-making," said jihadism expert Aymenn al-Tamimi.
"However, I think as with any government there is some autonomy in decision-making granted to more local manifestations of IS government departments."
-Who orders attacks?
There is not yet any hard evidence that top leaders directly ordered the Paris attacks, and it is likely that Baghdadi dictates strategies while less senior commanders and operatives see to the execution.
"We don't have any evidence at the present time that the Paris attack was ordered by the highest echelons of the Islamic State," Tamimi said.
"Baghdadi likely does not micromanage every attack that (IS) launches abroad. It is more likely that (IS) commanders execute campaigns according to Baghdadi's intent," said Harleen Gambhir, an analyst at the Institute for the Study of War.
IS-linked "operatives have attempted to attack Europe repeatedly this year. The Paris attacks simply represented a success in that effort, rather than a unique event that would have required special permission from Baghdadi," Gambhir said.
-How are fighters recruited?
IS employs a sophisticated propaganda machine to produce videos, photos and statements about its activities, including brutal public executions, helping it attract recruits. A large number of supporters disseminate this material online.
But for an operation like Paris, having operatives with connections in Europe is key.
IS "likely chose to use European foreign fighters to launch the Paris attacks because those fighters maintain connections to radical and criminal networks in their home countries," said Gambhir.
"These networks aid with the weapons procurement and other logistics required for an operation like the Paris attacks," she said.
-How is IS financed?
In territory it controls in Iraq and Syria, IS finances itself through means including oil smuggling, extortion, kidnapping for ransom and selling looted antiquities.
But external operations would largely be financed independently, though they may still receive some funding from the central IS organisation.
IS "is a different can of worms than Al-Qaeda traditionally has been," said Matthew Levitt, a former deputy assistant secretary for intelligence and analysis at the US Treasury Department who is now at the Washington Institute.
This is because it "controls a large swathe of territory, and is therefore able to do everything from tax people, both literal taxes and other taxes that we might think of more properly as forms of extortion, and all kinds of criminal activity," Levitt said.
But "the type of operations we just saw in Paris are mostly going to be independently financed, largely through criminal activity," he said.
-How much do attacks cost?
Not much, especially compared with the vast amounts of money spent on responding to and trying to prevent them. The exact figure depends on the number of people involved in the attack itself and in supporting it.
"We're not talking about a lot of money -- these things are cheap," said Levitt.
The Paris operation might have been pulled off for less than $50,000 (46,900 euros), he said, though that depends on exactly how many people took part, and it could have cost more.