Terrorism and counterterrorism have always been challenging subjects to study. Emotive, controversial and sometimes even dangerous, throughout the 20th century the study of both has lurked on the fringes of scientific research. There was a massive surge in research interest on terrorism and counterterrorism. Terrorism and counterterrorism entered university course curriculums across the globe and became widely taught. A variety of factors drive this fear of failure, but the reason most often cited is that the research methods used to study terrorism and counterterrorism are too weak. Persistent concerns about the impact of how terrorism was defined within research partly contributed to the development of the sub-field of critical terrorism studies (CTS) in the 2000s. Terrorism is an emotive subject, and many researchers have traditionally not been overly concerned with remaining objective and neutral in how they view the subject and its perpetrators. The chapter also presents an overview of the key concepts discussed in this book.