Political extremism is widely considered to be the product of irrational behavior. 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 which all insist on the rationality of extremism. Contributors use variants of this approach to shed light on subjects such as the conditions under which democratic parties take extremist positions, the relationship between extremism and conformism, the strategies adopted by revolutionary movements, and the reasons why extremism often leads to violence. The authors identify four core issues in the study of the phenomenon: 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.