The distinguishing feature of this collection by well-known economists and political scientists from North America, Europe, and Australia is to propose a variety of explanations that all insist on the rationality of extremism. The authors identify four core issues in the study of extremism: the nature (definition) of extremism and its origins in both democratic and authoritarian settings, the capacity of democratic political systems to accommodate extremist positions, the strategies (civil disobedience, assassination, lynching) chosen by extremist groups, and the circumstances under which extremism becomes a threat to democracy.