Risk assessment for terrorist threats is a field of growing importance throughout the world. A number of different approaches and methods are used for terrorist risk assessment based on a variety of philosophically and practically different assumptions and starting points. However, the foundations of these approaches and methods are often poorly understood and poorly articulated by those conducting terrorist risk assessment, yet these issues underpin success in managing risk of terrorist attacks. This book contributes to an improved understanding and consideration of foundational issues underpinning assessment and management of terrorist risks.