Both deterrence theory and deterrence practice are evolving to address contemporary strategic challenges. In the military domain, states progressively integrate and synchronise military operations. Outside of it, they exploit grey zone strategies that combine different instruments of influence across multiple domains. These developments are now giving birth to a new wave of thinking about cross domain deterrence (CDD), what it precisely entails, and what favouring conditions are necessary for it to be effective. This chapter situates CDD in the context of today’s challenges, and identifies the prerequisites for these favouring conditions based on a review of a rather diverse body of literature. It finds that one strand of that literature predominantly focuses on practical and technical prerequisites in order for CDD to be effective, leaving the framework of traditional deterrence theory intact. It also finds a second strand that holds that the nature of today’s challenges requires more than mere innovation in application. The ideas about deterrence proposed by this second strand are expanding on common understandings of deterrence to the extent that deterrence is no longer only about fear nor about convincing opponents to refrain from certain behaviour. The conclusion summarises the findings and elaborates their implications for theory and practice.