This important book examines why terrorism prevails in the otherwise stable and advanced democracies of Western Europe and why some countries have been more severely hit than others. Whilst Western Europe today seems relatively peaceful, some countries in this region have, in fact, experienced significantly high levels of terrorism for decades. Moreover, the threat has not only come from international terrorists operating in Europe but as a result of internal conflicts which have produced terrorist campaigns conducted by groups originating in the countries themselves. The author maps the trends in internal terrorism in 18 Western European countries since 1950 and explains those trends, both from a theoretical and empirical perspective. He uses a unique data set called TWEED, which covers around 9000 terrorist attacks and records the activities of about 200 terrorist groups over the post-war period. Offering a historical and comparative approach to terrorism, unlike the more usual focus on contemporary threats and developments, this book will appeal to political and social scientists and students, especially those working in comparative politics or on the causes of conflict. Academics interested in European studies and more specifically the conditions and developments of European democracy, and policymakers concerned with the development of the terrorist threat in Europe will also find the book of great interest.