Despite the democratic waves of the past decades, the 20th century was at least as much a century of political extremism. Even today, liberal democracy is increasingly challenged within its traditional heartlands of North America and Western Europe. This Major Work will bring together key papers on political extremism and radicalism, focussing predominantly upon extremism and radicalism within (liberal) democratic contexts, but also moving beyond this by including both (particularly historic) studies of the main extremist regimes and articles of the 20th century. Curated by a leading voice in the field, the articles are mapped and set in context by the introductory chapters which open each of the work's four volumes. Each volume focuses on a key area in the topic: Volume One: Extremism and Democracy: Concept, Theories and Responses Volume Two: Historical Extremism Volume Three: Right-Wing Extremism Volume Four: Left-Wing Extremism