This book explores the use of the internet by (non-Islamic) extremist groups, drawing together research by scholars across the social sciences and humanities. It offers a broad overview of the best of research in this area, including research contributions that address far-right, (non-Islamic) religious, animal rights, and nationalist violence online, as well as a discussion of the policy and research challenges posed by these unique and disparate groups. It offers an academically rigorous, introductory text that addresses extremism online, making it a valuable resource for students, practitioners and academics seeking to understand the unique characteristics such risks present.