Globally, the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific, and Cultural Organization's (IOC UNESCO) oversees the coordination of the global tsunami warning and mitigation system. Presently, there are 4 systems, each coordinated through Intergovernmental Coordination Groups (ICG) comprised of Member States or Countries. The systems are: Pacific Tsunami Warning and Mitigation System (ICG/PTWS), Tsunami and other Coastal Hazards Warning System for the Caribbean and Adjacent Regions (ICG/CARIBE-EWS), Indian Ocean Tsunami Warning and Mitigation System (ICGIOTWMS), and Tsunami Early Warning and Mitigation System in the North-eastern Atlantic, the Mediterranean and Connected Seas (ICG/NEAMTWS).

The ICG/PTWS was established in 1965 after the M9.5 Chile earthquake and tsunami, and the ICG/IOTWMS, ICG/CARIBE-EWS, and ICG/NEAMTWS were established in 2005 after the M9.1 Indonesia earthquake and Indian Ocean Tsunami. Each ICG meets regularly to coordinate international tsunami threat services and other mitigation activities.

In each region, event information services are provided by Tsunami Service Providers (TSP). The TSPs provide guidance to Country Tsunami Focal Warning Points (TWFP) and their National Tsunami Warning Centers (NTWC). Each country is solely responsible for determining the alert level (such as warning, advisory, watch) for their coasts using the international information and/or their own national assessment.

The current Tsunami Service Providers are:

CARIBE-EWS:  
Pacific Tsunami Warning Center 

IOTWMS:  
Joint Australian Tsunami Warning Centre (JATWC)
Indian National Centre for Ocean Information Services (INCOIS)
Meteorological, Climatological and Geophysical Agency of Indonesia (BMKG)

NEAMTWS:  
France CENtre d'Alerte aux Tsunamis (CENALT)
Greece Hellenic National Tsunami Warning Center National Observatory of Athens (NOA)
Turkey Kandili Observatory and Earthquake Research Institute of Istanbul (KOERI)
Italy Centro di Allerta Tsunami at Istituto Nazionale di Geofisica e Vulcanologia (IGNV)
Portugal Institute of the Sea and the Atmosphere (IPMA) (candidate)

PTWS:  
Pacific Tsunami Warning Center (PTWC) (https://tsunami.gov)
Japan Meteorological Agency Northwest Pacific Tsunami Advisory Center
Tsunami Warning Center of Ministry of Natural Resources, China                                                      Central America Tsunami Advisory Center (CATAC, candidate)

United States Tsunami Warning Centers (NOAA National Weather Service):
The USA operations two tsunami warnings centers which have different areas of responsibility.  Each center is a backup to the other. 

The USA National Tsunami Warning Center (US NTWC) in Palmer, Alaska, serves the continental United States, Alaska and Canada issuing Alerts (Warning, Advisories, Watches, and Information Statements).

The Pacific Tsunami Warning Center (PTWC) in Honolulu, Hawaii, directly serves the Hawaiian Islands, the US Pacific and Caribbean territories and commonwealths, and the British Virgin Islands issuing Alerts (Warning, Advisories, Watches, and Information Statements), and is the primary TSP for the Pacific and the sole TSP for the Caribbean issuing threat information to national authorities.