As the war on terrorism wages on, our nation's policymakers will continue to face the challenge of assessing threats that various terrorist groups pose to the U.S. homeland and our interests abroad. In addition, the struggle against terrorism likely will be in constant competition with other U.S. international policy issues that come to the fore. As part of the RAND Corporation's yearlong "Thinking Strategically About Combating Terrorism" project, the authors of this report develop a way to assess, analyze, and prioritize the danger posed by various terrorist organizations around the world. The authors also look at how different terrorist groups adapt and change over time, emphasizing that understanding these changes may help policymakers identify terrorists' greatest vulnerabilities. Of course, the very nature of terrorism creates a difficulty in predicting new and emerging threats; however, by establishing these types of parameters, the report creates a fresh foundation of threat analysis on which future counterterrorism strategy may build.