This introduction presents an overview of key concepts discussed in the subsequent chapters of this book. The book explains the literature on diffusion dynamics of political violence. It presents the cases ranging from the anarchist movement and left-wing militancy in European history to contemporary Islamist violence, to insurgent movements in South Africa and Latin America. These cases converge around four process-dynamics which we consider of major importance to processes of radicalization and the escalation of violence: dynamics of movement-state interaction; dynamics of intra-movement competition; dynamics of meaning formation and transformation; and dynamics of diffusion. Processes of radicalization conforms the understanding of social processes. It is influenced by a multitude of mechanisms and factors ranging from changes in opportunity structure and other macro-level factors to changes in predispositions and motives at the micro-level which may be grouped in different ways. Intra-movement interactions can entail mechanisms of radicalization that contribute to the emergence and increase of political violence.