This article reviews the academic literature on how and for what purposes violent extremists use the Internet, at both an individual and organizational level. After defining key concepts like extremism, cyber-terrorism and online radicalization, it provides an overview of the virtual extremist landscape, tracking its evolution from static websites and password-protected forums to mainstream social media and encrypted messaging apps. The reasons why violent extremist organizations use online tools are identified and evaluated, touching on propaganda, recruitment, logistics, funding, and hacking. After this, the article turns to the ways violent extremist individuals use the Internet, discussing its role as a facilitator for socialization and learning. The review concludes by considering the emergent literature on how violent extremism is being countered online, touching on both defensive and offensive measures.