This study evaluates whether surprise and intelligence failure leading to mass casualty terrorism are inevitable. It explores the extent to which four factors – failures of public policy leadership, analytical challenges, organizational obstacles, and the inherent problems of warning information – contribute to intelligence failure. The study applies existing theories of surprise and intelligence failure to case studies of five mass casualty terrorism incidents: World Trade Center 1993; Oklahoma City 1995; Khobar Towers 1996; East African Embassies 1998; and September 11, 2001.