Military rapid response mechanisms are generally understood as troops that are on standby, ready to be deployed to a crisis within a short time frame. Despite the general recognition about the added-value of military rapid responses, there is still much debate about who should undertake such rapid reaction missions. While inter-organizational relations are present in the entire crisis management cycle, the institutional proliferation in rapid response mechanisms implies that questions of cooperation and competition between these organizations are an increasingly relevant concern in the phase of military rapid response. The effects of the uncertainty on international organizations’ capacity to actually rapidly deploy troops, as well as on the inter-organizational dynamics of military rapid response in-theater, have to date hardly been studied. The chapter also presents an overview of the key concepts discussed in this book.