In this volume in the MIT Press Essential Knowledge series, J. M. Berger offers a nuanced introduction to the extremist movements threatening to destabilize civil societies around the globe. He explains what extremism is, how extremist ideologies are constructed, and why extremism can escalate into violence. Berger shows that although the ideological content of extremist movements varies widely, there are common structural elements.

Berger describes the evolution of identity movements and individual and group radicalization, offering case studies and examples such as:

- The destruction of Carthage by the Romans--often called "the first genocide"
- The apocalyptic jihadism of Al Qaeda
- America's new "alt-right"
- The anti-Semitic conspiracy tract The Protocols of the Elders of Zion

Berger, an expert on extremist movements and terrorism, explains that extremism arises from a perception of "us versus them," intensified by the conviction that the success of "us" is inseparable from hostile acts against "them." Extremism differs from ordinary unpleasantness--run-of-the-mill hatred and racism--by its sweeping rationalization of an insistence on violence. If we understand its causes and the common elements of its movements, Berger says, we will be more effective in countering it.