The United States seems to be more ideologically divided than ever, with political polarization at an all-time high in recent history. Political differences can be positive, as when they drive an increase in political engagement or push both sides to come to better conclusions. But taken to an extreme, they can be dangerous. The viewpoints in this volume examine extremists on both the right and the left, how domestic extremism has evolved since the advent of the internet and social media, and how law enforcement at various levels should address it, particularly in the prevention of extremist attacks.