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.
National Counter-Terrorism Alert Level
Low - terrorist attack is not expectedMedium - terrorist attack could occurHigh - terrorist attack is likelyExtreme - terrorist attack is imminent or has occurredAdministered by the Federal Attorney-General's Department.
Plan Vigipirate
Yellow - to stress vigilanceOrange - to warn of terrorist actionRed - to warn of serious attemptsCrimson - to warn of major attemptsIreland (Republic of)
International Terror Threat Level
Low: An attack is deemed unlikely.Moderate: An attack is possible, but not likely.Substantial: An attack is a strong possibility.Severe: An attack is highly likely.Critical: An attack is imminent.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".
Blue - an unconfirmed information about real terroristic threatYellow - an confirmed information about real terroristic threatRed - an information about committed terrorist actLow - an attack is unlikelyModerate - an attack is possible, but not likelySubstantial - an attack is a strong possibilitySevere - an attack is highly likelyCritical - an attack is expected imminentlyAdministered by the Joint Terrorist Analysis Centre
Homeland Security Advisory System
Low - Low risk of terrorist attacksGuarded - General risk of terrorist attacksElevated - Significant risk of terrorist attacksHigh - High risk of terrorist attacksSevere - Severe risk of terrorist attacksAdministered by the Department of Homeland Security
National Terrorism Advisory System
Elevated Threat - Significant or high risk of terrorist attacksImminent Threat - Critical risk of terrorist attacks