Terror alert systems are standardised emergency population warning systems for describing and disseminating information about terrorism-related threats. They became more popular after the September 11 attacks on the United States in 2001.

Australia

National Counter-Terrorism Alert Level Administered by the Federal Attorney-General's Department.

France

Plan Vigipirate

Ireland (Republic of)

International Terror Threat Level Administered by the Garda Síochána. Since November 2015, the international terrorism threat level is considered to be at moderate, meaning an attack is "possible but not likely".

Netherlands

Russia

United Kingdom

Administered by the Joint Terrorist Analysis Centre

United States

Homeland Security Advisory System Administered by the Department of Homeland Security National Terrorism Advisory System