Polling by the Chicago Council on Global Affairs reveals that record numbers of Americans now consider terrorism directed or inspired by ISIL and Al Qaeda to be a critical threat to the United States. In Europe, the scale of recruitment of “foreign fighters” and the frequency of terror attacks has shown this challenge to be even more acute. Attacks in Belgium, France, and Germany have revealed the challenge that governments face in countering radicalization that leads to terrorism, and dealing with terrorist operatives returning from conflict areas. How have Europe’s counter-terrorism strategies evolved in response to ISIL? What can the United States and Europe learn from each other’s response to this transnational threat?