A state of emergency is a governmental declaration that may suspend some normal functions of the executive, legislative and judicial powers, alert citizens to change their normal behaviours, or order government agencies to implement emergency preparedness plans. It can also be used as a rationale for suspending rights and freedoms, even if guaranteed under the constitution. Such declarations usually come during a time of natural or man made disaster, during periods of civil unrest, or following a declaration of war or situation of international or internal armed conflict. Justitium is its equivalent in Roman law.

In some countries, the state of emergency and its effects on human rights and freedoms and governmental procedure are regulated by the constitution and/or a law that limits the powers that may be invoked. Rights and freedoms may be suspended during an emergency, for instance, freedom of movement, but not non-derogable rights. In many countries it is illegal to modify the emergency law or the constitution during the emergency.