Civil defence (or civil defense) is a system used to protect the civilians of a state from coming under attack. It involves plans for preparing for and responding to emergency situations, plans for evacuation, and plans for how to recover from attacks. In some countries, civil defence is seen as an important part of military defence. Programmes of this kind were first explored at least as early as the 1920s. They have been used in many countries, especially since the threat of nuclear weapons has become known.
Since the end of the Cold War, the focus of civil defence has become more and more for disasters in general. The new concept is described by a number of terms, each of which has its own specific meaning, such as crisis management, emergency management, emergency preparedness, emergency services, and civil protection.