Estimates of the number of Muslims in Europe vary widely, depending on the methodology and definitions used, and the geographical limits imposed. Excluding Turkey and the Balkans, researchers estimate that as many as 15 to 20 million Muslims live on the European continent. Muslims are the largest religious minority in Europe, and Islam is the continent's fastest growing religion. Substantial Muslim populations exist in Western European countries, including France, Germany, the United Kingdom, Spain, Italy, the Netherlands, and Belgium. Most Nordic and Central European countries have smaller Muslim communities. Europe's Muslim population is ethnically and linguistically diverse; Muslim immigrants hail from Middle Eastern, African, and Asian countries, as well as Turkey. Many Muslim communities have their roots in Western European labor shortages and immigration policies of the 1950s and 1960s that attracted large numbers of North Africans, Turks, and Pakistanis especially. In recent years, there have been influxes of Muslim migrants and political refugees from other regions and countries, including the Balkans, Iraq, Somalia, and the West Bank and Gaza Strip. Historically, European countries have pursued somewhat different policies with respect to managing their immigrant and minority populations. However, none has been completely successful. A disproportionately large number of Muslims in Europe are poor, unemployed, or imprisoned, and many feel a sense of cultural alienation and discrimination. For decades, countries such as Germany, Austria, and Switzerland viewed Muslim immigrants as temporary "guest workers." As a result, little effort was made at integration, and parallel societies developed. Britain and the Netherlands embraced the notion of multiculturalism--integration while maintaining identity--but in practice, this concept helped entrench discrete Muslim communities, functioning apart from the culture of the host country. Nor has France's assimilation policy prevented the segregation of its Muslim communities, as seen most vividly in the riots that erupted throughout France in the fall of 2005 in working class suburbs populated largely by North Africans. The protests in several European cities in early 2006 sparked by the publication in European newspapers of cartoons of the prophet Muhammad also highlight the disaffection and alienation that many European Muslims feel. Although the vast majority of Muslims in Europe are not involved in radical activities, Islamist extremists and fringe communities that advocate terrorism exist and reportedly have provided cover for terrorist cells. Europe's largely open borders and previously non-existent or lax terrorism laws have also allowed some Islamist terrorists to move around freely. Following the September 11, 2001 attacks on the United States, Germany and Spain were identified as key planning bases; numerous terrorist arrests were also made in Belgium, France, Italy, and the UK. The March 11, 2004, bombings of commuter trains in Madrid, Spain that killed 191 people were carried out by an Al Qaeda-inspired group of North Africans, mostly Moroccans resident in Spain. Even before terrorists struck London's mass transport system in July 2005, many analysts believed that the UK had become a breeding ground for Islamist extremists. Radical mosques in London apparently indoctrinated Richard Reid, the airplane "shoe bomber," and Zacarias Moussaoui, the "20 th " September 11 hijacker. UK authorities have named four young British Muslims as the perpetrators of the July 7, 2005 London attacks that killed 52 people, plus the four bombers, and injured over 700. Three of the alleged bombers were of Pakistani descent and had recently traveled to Pakistan, where some suspect they received terrorist training from remaining Al Qaeda operatives. On July 21, 2005, four Muslim immigrants tried but failed to set off four other explosions on London's metro and bus lines; no casualties resulted. And in August 2006, British police arrested several British Muslims suspected of involvement in a plot to detonate liquid explosives on airliners flying from the UK to the United States. Nationals aligning their beliefs with Al Qaeda or radical Islam are not unique to Europe. The United States has captured or identified several U.S. citizens with similar views in the course of the fight against terrorism. However, some assert that the failure of European governments to fully integrate Muslim communities into mainstream society leaves some European Muslims more vulnerable to extremist ideologies. Many experts say that some European Muslim youth, many of whom are second or third generation Europeans, feel disenfranchised in a society that does not fully accept them; they appear to turn to Islam as a badge of cultural identity, and are then radicalized by extremist Muslim clerics. Traditionally liberal asylum and immigration laws in Western Europe, as well as strong free speech and privacy protections, have attracted numerous such clerics and Middle Eastern dissidents. Some experts also believe that the recent wars in Afghanistan and Iraq have radicalized more European Muslims, and strengthened terrorist recruitment efforts. Many European Muslims claim common cause with suffering brethren in the Israeli-occupied Palestinian territories, as well as in Iraq, Chechnya, and elsewhere. They tend to view the "war on terrorism" as a war on Islam, and perceive an unjust double standard at work in the foreign policies of many European governments, especially those that supported the U.S.-led war in Iraq. Before being captured in September 2005, Al Qaeda training camp manager Abu Musab al-Suri noted in a communique that he had overseen the training of both Arab and non-Arab Muslims, including some individuals born or raised in Britain, the United States, and other Western countries. Al-Suri called upon the mujahideen in Europe to act quickly and strike the UK, the Netherlands, Italy, Denmark, Germany, France, and other countries with a military presence in Iraq, Afghanistan, or on the Arabian peninsula. Media reports indicate instances of French Muslim teenagers being recruited to fight in Iraq; German, Italian, and Spanish law enforcement authorities also report that they have disrupted efforts by Islamist extremists to recruit European youths for Iraq. In November 2005, a Belgian woman and convert to Islam blew herself up in an attempted suicide attack on U.S. forces in Iraq. Some analysts suggest that religious converts to Islam may be more susceptible to radicalization as a result of a mistaken desire to prove themselves in their new faith. Others note that Europe's physical location--within a few days driving distance to Iraq or Chechnya--makes it vulnerable to fighters returning from conflict zones who have either European roots or are unable to return to their countries of origin. Press reports suggest that Iraqis and others tied to the insurgency have been active in Europe. In December 2004, German authorities arrested three Iraqis suspected of plotting to assassinate the interim Iraqi prime minister during a visit to Berlin; the three Iraqis allegedly belong to Ansar al-Islam, which has organized strikes against U.S. troops and others in Iraq. In January 2006, a German court convicted an Iraqi man of both recruiting young men for the Iraqi insurgency and smuggling extremists from Iraq into Germany, Britain, and other European countries. Central and Eastern Europe has not been reported to be as important a haven for Al Qaeda and other terrorist groups; most countries in the region have not attracted significant numbers of Muslim immigrants. However, concerns have been raised about several countries in Southeastern Europe with large Muslim populations (e.g., Bosnia-Herzegovina). One legacy of the 1992-1995 war in Bosnia is the presence of Islamist fighters from other countries who stayed behind and became Bosnian citizens. Some Islamic charities that proliferated during and after the war reportedly served as Al Qaeda money-laundering fronts. Terrorist groups have also operated from Albania. At the same time, opposition to terrorism among indigenous Muslims in the Balkans has been strong. Most view themselves as part of Europe and are grateful for the U.S. role in defending them against Serbian aggression in the 1990s. U.S. officials say that efforts by Islamist extremists to recruit local Muslims have met with limited success, and they praise these countries' anti-terrorism efforts, especially after September 11. Nevertheless, some experts assert that Central and Southeastern Europe may pose a more significant threat than often acknowledged. The region's weak governing institutions and problems with organized crime and corruption may make it vulnerable to infiltration by terrorist groups. Observers caution that the Balkans in particular may play a role as a transit point for terrorists, a target area for recruitment, and a potential source of weapons or explosives. The November 2004 murder of Dutch filmmaker Theo van Gogh brought the issue of Islamist extremism in Europe to the forefront of European political debate. Van Gogh, an outspoken critic of the treatment of women in Islam, was killed by a 27-year-old Dutch citizen of Moroccan descent and a follower of radical Islam. Since the murder, many European officials and social commentators have proclaimed that multiculturalism in Europe has failed, and called for greater integration of Muslims and other immigrants into mainstream European society. They believe that Muslims and others must embrace the native cultures of their new countries, including secularism. Some European governments have been pursuing initiatives aimed at fostering integration and promoting secularism for several years. The French government, for example, has banned "conspicuous" religious symbols in public schools, including headscarves for Muslim girls, yarmulkes, and large crucifixes. Moderate Muslim groups in France supported the ban as a means to reduce tensions in the school system and in broader society. The UK is introducing new citizenship classes to ensure that immigrants can speak English and understand British history and culture. Other analysts say that countries such as Spain, Italy, and Germany need to do more to encourage Muslim immigrants to become citizens. Some European governments are trying to encourage moderate Muslim political voices and promote a greater role for them. Commentators note, for example, that there are few Muslim representatives in European parliaments. In 2003, Paris established an elected French Council of the Muslim Faith, an official advisory body that acts as the Muslim community's representative in dealings with the French government. French and British officials are also looking at ways to foster "homegrown imams" to minister to the needs of their Muslim communities, rather than relying on foreign imams whom they claim are often unfamiliar with the West or beholden to foreign interests. The Netherlands has reportedly created an "imam buddy system" that links foreign imams with Dutch volunteers to promote a better understanding among these imams of Dutch culture and society. Others argue that greater focus should be placed on addressing the lack of jobs and educational opportunities for Muslims, as well as racism. They say that racial violence against Muslims is on the rise in some European countries, such as the Netherlands, but governments have failed to acknowledge the scale of the problem. Several analysts suggest that mainstreaming Muslims into European society would not necessarily translate into an embrace of European ideals; some even question whether Islam itself is compatible with European political principles and values. They point out, for example, that two British Muslim suicide bombers in Israel in April 2003 were from comfortable middle-class, Westernized suburbs. Some Muslim groups in Europe say that certain efforts toward integration, such as the French headscarf ban, are counterproductive and only serve to increase the sense of discrimination among Muslims. In the wake of the London bombings, the UK government is consulting with British Muslims on how to best tackle extremism. Some Muslim leaders argue that Muslim communities must be more vocal against extremism, and actively counter rather than tolerate radical preachers. European governments have also sought to contain Islamist extremists and counter terrorists by tightening security measures and reforming immigration and asylum laws. UK and French security services have reportedly increased their monitoring of mosques; Germany has changed its laws to allow authorities to investigate religious groups; and France and Italy have expelled some Muslim clerics for hate crimes. Following the London attacks in 2005, the British government has sought to make it easier to exclude or deport foreign individuals who incite hatred. Also notable are European Union (EU) efforts to boost police and judicial cooperation, enhance intelligence-sharing, and strengthen external EU border controls. Security and border control services in new EU members in Central and Eastern Europe, although not quite as effective as their Western counterparts, are also improving as they seek to meet EU standards. In addition, the EU has been working to encourage good integration practices among its 27 member states and prevent radicalization. Law enforcement challenges remain throughout Europe, as elsewhere. Long-standing traditions against intelligence-sharing, rivalries among the various local and national security services, and different national laws continue to impede more robust EU cooperation. For example, full implementation of the EU-wide arrest warrant has been slowed in Poland and Cyprus because of court rulings that found the warrant incompatible with constitutional bans on extraditing their own nationals. European governments are also struggling with balancing their efforts to curtail Islamist extremists against well-established civil liberty protections, strong privacy rights, and democratic ideals. U.S. officials have expressed concerns since the 2001 terrorist attacks that Europe might be a launching point for future attacks on the United States and U.S. interests abroad. The Bush Administration and Members of Congress have welcomed European initiatives to curtail Islamist extremism and improve U.S.-EU counterterrorism cooperation in the hopes that such efforts will ultimately help root out terrorist cells in Europe and beyond. The United States and the EU have been placing increasing emphasis on cooperation in the areas of intelligence-sharing, border control, and transport security. Among other initiatives, the two sides have concluded agreements to improve container security and exchange airline passenger information. Nevertheless, some challenges remain; for example, differences persist in U.S.-EU data protection regimes and, at times, have complicated closer cooperation on travel security. Some terrorism experts and Members of Congress remain concerned about the U.S. Visa Waiver Program (VWP), despite steps taken to tighten passport requirements for participating countries. The VWP allows more than 15 million people a year short-term visa-free travel to the United States from 27 countries, most of which are in Europe. The VWP has become a sticking point in U.S.-EU relations; the EU would like the VWP to be extended to all EU members (currently 12 are excluded due to problems meeting U.S. immigration laws). Some Members of Congress oppose expanding or even continuing the VWP, noting that Islamist terrorists who hold European citizenship have entered the United States on the VWP (UK-born Richard Reid and French citizen Zacarias Moussaoui being two notable examples). Also, stolen passports from VWP countries are prized travel documents among terrorists, criminals, and immigration law violators, creating an additional risk. Other Members are more supportive of extending the VWP to new EU members, mostly in central and eastern Europe, given their roles as U.S. allies in NATO and in the fight against terrorism. Some experts also caution that eliminating or curtailing the VWP could impede transatlantic tourism and commerce. Some analysts contend that the presence of large Muslim communities in Europe may also be influencing the policy preferences of some European governments on contentious Middle East issues and contributing to U.S.-European divisions. They argue that Europe's growing Muslim population has made some European officials more cautious about supporting U.S. policies that risk inflaming their own "Muslim streets." They suggest that this is one reason why countries such as France and Germany opposed the U.S.-led war in Iraq. Meanwhile, Europe's struggle with its own identity as it grapples with integrating Muslims into European society has called into question Turkey's long-term EU prospects. Washington has long advocated EU membership for Turkey, a country of 70 million Muslims, as a way to anchor this strategic ally firmly in the West and debunk the notion of a clash of civilizations between Islam and the West.

Summary:
Although the vast majority of Muslims in Europe are not involved in radical activities, Islamist extremists and vocal fringe communities that advocate terrorism exist and reportedly have provided cover for terrorist cells. Germany and Spain were identified as key logistical and planning bases for the September 11, 2001 attacks on the United States. The March 2004 terrorist bombings in Madrid have been attributed to an Al Qaeda-inspired group of North Africans. UK authorities have named four British Muslims as the perpetrators of the July 2005 terrorist attacks on London; in August 2006, British law enforcement arrested several British Muslims suspected of plotting to blow up airliners flying from the UK to the United States. This report provides an overview of Islamist extremism in Europe, possible terrorist links, European responses, and implications for the United States. It will be updated as needed. See also CRS Report RL31612, European Counterterrorist Efforts: Political Will and Diverse Responses in the First Year After September 11, by [author name scrubbed] (pdf), and CRS Report RL33166, Muslims in Europe: Integration in Selected Countries, by [author name scrubbed] et al.