Civil defense (British English: civil defence) or civil protection is an effort to protect the citizens of a state (generally non-combatants) from man-made and natural disasters. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation and recovery. Programs of this sort were initially discussed at least as early as the 1920s and were implemented in some countries during the 1930s as the threat of war and aerial bombardment grew. Civil-defense structures became widespread after authorities recognised the threats posed by nuclear weapons.

Since the end of the Cold War, the focus of civil defense has largely shifted from responding to military attack to dealing with emergencies and disasters in general. The new concept is characterised by a number of terms, each of which has its own specific shade of meaning, such as crisis management, emergency management, emergency preparedness, contingency planning, civil contingency, civil aid and civil protection.

Some countries treat civil defense as a key part of defense in general. For example, the Swedish-language word totalförsvar ("total defense") refers to the commitment of a wide range of national resources to defense, including the protection of all aspects of civilian life. Some countries have organized civil defense along paramilitary lines, or have incorporated it within armed forces, such as the Soviet Civil Defense Forces (Войска гражданской обороны).