This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book provides an overview of the ways in which the academic literature has tended to characterize the role of fear and psychological violence in the process of political terrorism. It explores issues involved in balancing public security against individual privacy. The book provides an overview of how the problems associated with rail trespass and suicide are being considered across Europe as well as examples from national prevention programmes that are implementing various interventions. It also provides an interdisciplinary framework from which to study the behaviour of militant groups that either carry out acts of political violence themselves or support the use of violence by others. The book presents a series of case studies in terrorism investigations focusing on the emerging threat of home-grown terrorism as well as individuals returning from conflict abroad.