The main objectives of the initiative are to: 1) Assemble and standardize existing information on MMEs; 2)Identify geographic gaps that need to be addressed through future monitoring and research efforts; 3) Assessing species vulnerability to MMEs and identifying observation taxonomic gaps; 4) Fostering the analysis of the relationship between MMEs and environmental conditions, with an emphasis on marine heat waves; 5) Provide an unrestricted, open-source and easy access dataset; 6) Ensure transparency and clarity regarding the origin of each dataset and adequate data citation; 7) Providing information for the assessment of the impact of MMEs on the biodiversity and socio-economic activities. Field observations of mass mortality events in a specific geographic location (site) are the core of the MME-T-MEDNet database. One database record corresponds to the observation of abnormal (high) values of partial and/or total mortality (usually through quantitative indicators) in one local population at one specific time (or period). Here, we consider a local population as a group of colonies or specimens/individuals of the same species (ranging from tens to thousands depending on the species) dwelling in a specific geographic location that is defined by coordinates and depth range.