By integrating the fragmented research on emergency services, armed forces, and humanitarian organizations, this book identifies the components of a new theory on frontline crisis response. To begin with, the work of responders is characterized by persistent operational dilemmas. Since there are no universal solutions, they need to adapt their approaches and decisions to the situational contingencies of a crisis. These adaptations continue throughout the crisis response process as the situation evolves. Responders usually pragmatically act their way through operational dilemmas in the crisis response process. These experiences nevertheless have an existential effect on their identities and lives. Thus, the new theory comprises operational dilemmas, situational contingencies, response processes, pragmatic principles, and existentialist ideas. This theory offers a basis for crisis response improvements and contributes to the literature on strategic crisis management, frontline work in organizations, reliable organizing in risky contexts, and post-crisis operations. The chapter ends with a research agenda and a call for more academic engagement with frontline crisis response.