Many commentators have noted the difficulty of conceptualizing and measuring terrorism. These difficulties can be seen on both conceptual and methodological levels. The chapter shows that conceptualizing and recording terrorist attacks is a complex task. To illustrate the complexity of conceptualizing and measuring terrorism in the rest of this chapter, it consider how the various elements of the original Pinkerton Global Intelligence Service (PGIS) definition of terrorism and the added criteria have been operationalized by the team that produces the Global Terrorism Database (GTD). On the methodological level, measuring terrorism is challenging because the main parties to terrorist attacks either lack accurate information, do not share a common view of what terrorism is, or have information that they are unwilling to share. Victimization surveys have been of little use in the study of terrorism. Despite the attention it gets in the global media, terrorism is much less common than more familiar types of violent crime like robbery and homicide.