The Intergovernmental Coordination Group for the Tsunami and Other Coastal Hazards Warning System for the Caribbean and Adjacent Regions at its eighth session (ICG/CARIBE EWS-VIII, Port of Spain, Trinidad and Tobago, 29 April–1 May 2013), decided to conduct tsunami exercises named CARIBE WAVE on an annual basis leaving it to each Member State to define its level of participation. The ICG/CARIBE-EWS Officers at their meeting on 6–8 May 2020 recommended that Exercise CARIBE WAVE 21 take place on 11 March 2021 at 14:00 UTC (Coordinated Universal Time) with the following two hypothetical scenarios: • A tsunami generated by a magnitude 8.0 earthquake located on the Enriquillo-Plantain Garden Fault Zone in Jamaica; • A tsunami generated by a magnitude 8.5 earthquake off the northern Lesser Antilles. The purpose of this exercise is to practice and evaluate the operations of the CARIBE-EWS Tsunami Warning System along the coasts of the Caribbean and adjacent regions. It provides an opportunity to evaluate both the CARIBE-EWS products of the US Pacific Tsunami Warning Center (PTWC) and the corresponding readiness of national emergency management organizations and communities at risk including operational lines of communications, tsunami response procedures, and evacuation procedures, and promote tsunami preparedness at community level. Regular exercising of response plans is critical to maintain readiness of the whole tsunami warning system in case of an emergency. In light of the Covid-19 pandemic, CARIBE WAVE 21 represents an opportunity to also integrate new or special procedures that Member States and territories can tailor according to the physical and social distancing measures in place. The ICG/CARIBE-EWS of UNESCO/IOC, the US National Oceanic and Atmospheric Administration (NOAA), and the Caribbean Regional Stakeholders (CEPREDENAC, CDEMA, and EMIZA) are providing the framework for this exercise as a means for emergency responders throughout the Caribbean and adjacent regions to test and update tsunami response plans. The participant handbook with details of the exercise is available online at http://www.caribewave.info (IOC Technical Series, 157, Vol.1). Additional information relevant to the exercise will also be posted on this link in due course. The Member States and territories involved in the exercise are invited to select one of the above mentioned scenarios by completing the online survey (https://forms.gle/NptfCm4xRMw4Bgrw6) by 26 February 2021. The US Pacific Tsunami Warning Center (PTWC will issue one initial dummy message on 11 March 2021 at 14:00 UTC and will disseminate it through all standard Tsunami Warning Centre broadcast channels. This dummy message shall be issued to test communications with Tsunami Warning Focal Points (TWFPs) and National Tsunami Warning Centres (NTWC). This first message will start the exercise. As of 14:07 UTC, the PTWC will send by email the simulated tsunami products corresponding to each scenario to officially designated TWFPs and NTWCs. Each TWFP/NTWC will decide if and how to disseminate the messages within their area of responsibility. In case where a Member State does not select one of the above scenarios, the organizers will unilaterally opt for one of the two scenarios. For CARIBE WAVE 21, the CARIBE-EWS organization team has teamed up with TsunamiZone.org for an online registration of individual participants to the exercise at http://tsunamizone.org/caribbean. Under the ‘Caribbean Zone Region’ tab, participants will be able to sign up as of January 2021, and receive an email confirming participation. National Emergency Management Organizations (EMOs) will have real-time access to the status of participants’ registration within their areas of responsibility. EMOs are encouraged to promote this registration system widely to the population concerned. During CARIBE WAVE 19, of almost 800,000 total participants, over 500,000 registered to the exercise through TsunamiZone.org. For CARIBE WAVE 20 exercise, 103,246 participants had signed up when the exercise was changed to a mere communication exercise due to the spread of Covid-19. Orientation webinars for participants are offered in English, Spanish and French languages over the periods 26–28 January and 23–25 February 2021. Registration to seminars is at http://www.caribewave.info. For further information on the exercise, please contact Dr Silvia Chacon Barrantes, Chair ICG/CARIBE-EWS (email: silviach@una.ac.cr), Dr Elizabeth A Vanacore, Chair of the Task Team CARIBE WAVE 20 (email: elizabeth.vanacore@upr.edu), Ms Alison Brome, Caribbean Tsunami Information Center (CTIC) (email: a.brome@unesco.org), Ms Christa G. von Hillebrandt-Andrade, Manager US NWS Caribbean Tsunami Warning Program (email: christa.vonh@noaa.gov), and/or the Technical Secretary ICG/CARIBE-EWS, Mr Bernardo Aliaga (email: b.aliaga@unesco.org). You are also encouraged to disseminate copies of this letter to appropriate organizations and authorities within your country. After the exercise each ICG/CARIBE-EWS Member State and territory is requested to provide a feedback on the exercise not later than 26 March 2021 using the online evaluation survey at https://www.surveymonkey.com/r/Caribe wave21. The survey will assist the preparation of an evaluation report that will be published as volume 2 of IOC Technical Series 157.