A civil defense siren is a siren used to provide an emergency population warning to the general population of approaching danger. Initially designed to warn city dwellers of air raids during World War II, they were later used to warn of nuclear attack and natural disasters, such as tornadoes. The generalized nature of sirens led to many of them being replaced with more specific warnings, such as the broadcast-based Emergency Alert System and the Cell Broadcast-based Wireless Emergency Alerts and EU-Alert mobile technologies.

An emergency population warning is a method where by local, regional, or national authorities can contact members of the public to warn them of an impending emergency. These warnings may be necessary for a number of reasons, including:Weather emergencies such as tornadoes, hurricanes, and ice storms. Geological disasters such as earthquakes, landslides, volcanic eruptions, and tsunamis. Industrial disasters such as the release of toxic gas or contamination of river water. Radiological disasters such as a nuclear plant disaster. Medical emergencies such as an outbreak of a fast-moving infectious disease. Warfare or acts of terrorism.