This policy report draws on the discussions during a meeting entitled “Colloquium on Measuring Effectiveness in Counterterrorism Programming” and held in Ottawa on 9–10 February 2012, as well as discussions with experts, government officials, and an initial desktop literature review. The colloquium was developed and organized by the Center on Global Counterterrorism Cooperation with the support of the governments of Canada, the Netherlands, and the United Kingdom. Participants included counterterrorism and terrorism prevention experts from national governments and international organizations, as well as academics and practitioners, in these and related fields. Participants agreed that the emphasis on program evaluation and terrorism prevention is timely and appropriate. Given the responsibility of demonstrating to their citizens the effectiveness of their terrorism prevention measures, governments are especially concerned to know which ones have the most impact.