This chapter examines the global threat posed by terrorism. Efforts to deal with terrorism can be considered within the framework of terrorism as warfare, terrorism as crime, and terrorism as disease. Which of these views is adopted often plays a role in determining what kinds of measures to use to counter terrorism. Terrorism is a technique of action available to many different groups; security measures that work with one group may not be effective with others. As a consequence, dealing with terrorism in today’s world can be a very complex process. The chapter first discusses concepts and definitions relating to terrorism before describing various types and causes of terrorism. It also analyses counterterrorism measures within the scope of prevention, response to attacks, international collaboration, and the effects of security. Three case studies involving the Palestinian Liberation Organization, the Ku Klux Klan, and the Irish Republican Army are presented.