It's a difficult question. Hamas is believed to receive funding from Palestinian expatriates, the Iranian government, and private donors in Saudi Arabia and other Arab states.
Charities in the United States, Canada and Western Europe have funded Hamas-linked social service groups. In addition, several individuals in the United States have been charged with providing material support to the organization.
Where does Hamas stand in the Palestinian power structure?
Hamas is essentially in opposition to the Palestinian Authority, although the late Palestinian leader Yasser Arafat was careful not to antagonize Hamas fearing the wrath of the organization's tens of thousands of supporters and sympathizers.
In the past, the Palestinian Authority has launched major offensives against the group, notably the arrests of nearly 1,000 Hamas supporters and the taking over of mosques in Gaza in 1996. But the Palestinian Authority has had to be careful not to drive Hamas underground and has made some attempts to co-opt the organization into mainstream Palestinian politics.
Why is Hamas popular?
Hamas is seen as an honest organization compared with the Palestinian Authority, which had a reputation for corruption -- especially during the brief period of relative peace after the Oslo Accords were signed in the mid-1990s.
The group's policy of opposing short-term agreements with Israel until there is a complete Israeli withdrawal from the Palestinian territories won many supporters after Oslo, when Israeli Prime Minister Ehud Barak implemented a massive expansion of Jewish settlements in the territories.
Hamas' popularity again soared during the second Palestinian intifada, which began in September 2000.