In order to enhance the development of an evaluation policy in the Netherlands, Nelen et al. provided policy makers in the field of counterterrorism with some building blocks for an evaluation framework. The chapter focuses on three elements of the evaluation framework. The first element is linked to the theories that underlie a specific policy, program, or intervention. Secondly, the multiagency aspects of counterterrorism policies are discussed. Third, the prevention and containment of terrorist activities imply that several agencies cooperate with each other by sharing information and integral enforcement. Evaluation studies and evaluation policy in general benefit from an open, professional relationship between researchers and their environment. In the process of assessing the outcomes of counterterrorism, a distinction can be made between the observed output—based on performance indicators, including the unexpected and counterproductive ones—of this output on terrorism. Theory-based evaluation is closely related to realist evaluations that stress the importance of knowing which mechanisms make policies work.