This book explores one of the most topical and controversial issues of recent years –jihadist terrorist infiltration of irregular migrant flows to Europe. Utilizing robust sampling criteria, more than a hundred such cases are identified and rigorously assessed. The analysis reveals the characteristics of offenders, their travel patterns and operational activities, and critically evaluates subsequent law enforcement and judicial responses. The author draws upon interviews with a range of European security officials, as well as non-governmental organization employees, and a recent refugee, in order to provide a series of practical recommendations.