While terrorist attacks are, by their very nature, surprise attacks, they are rarely studied as such. There have been few attempts to integrate knowledge and insight from the extensive bodies of literature on military surprise attack and terrorism. This article proposes a framework for understanding the relationship between the mechanism of surprise and the method of terrorism. It seeks to situate the principle of surprise within the tactical and strategic logic of terrorism in order to illuminate the role of surprise as the terrorist's tactical mechanism of necessity and his strategic weapon of choice. Applying this framework to the 9/11 case will further illustrate the central role of surprise in terrorism.