An overview of response management in the early phase of an emergency at a nuclear installation is provided from a systems approach. This starts with the recognition of response goals, and using detailed analyses of threats, past experience, international law and principles, a response strategy is developed. The process is illustrated for the case of severe accidents at pressurized water reactors (PWRs) and identifies the need for and nature of: emergency classification based on plant conditions, notification, radiological monitoring and assessment strategies, operational criteria for implementing protective action decisions and management of public information. From the strategy detailed, functional and infrastructure requirements can be defined. The paper also presents some reflections on the key differences between response to emergencies arising from accidents and those arising from deliberate acts; on the preparedness and response capabilities of States, highlighting areas where generic improvements are needed and how best to achieve that.