This chapter looks into the rationality of terrorism. It starts off by looking into the paradox of terrorism. Political scientists typically view terrorists as rational political actors. However, empirical research on terrorism suggests that terrorism is in fact an ineffective political tactic. Evidence indicates that in instances where there has been terrorist attacks on civilians, governments rarely grant concessions. This might explain why terrorism is often selected as a tactic only if alternative options are no longer viable. The chapter uses Al-Qaeda and the Islamic State as case studies to examine broader patterns of terrorism. Knowing the priority of terrorists is vital for governments when considering counterterrorism actions. Having an understanding of the grievances of terrorists helps political actors predict which targets the terrorists will attack.