ODYSSEA will develop, operate and demonstrate an interoperable and cost-effective platform that fully integrates networks of observing and forecasting systems across the Mediterranean basin, addressing both the open sea and the coastal zone. The platform will collect its data from the many databases maintained by agencies, public authorities, and institutions of Mediterranean EU and non-EU countries, integrating existing earth observation facilities and networks in the Mediterranean Sea building on key initiatives such as Copernicus, GEOSS, GOOS, EMODNet, ESFRI, Lifewatch, Med-OBIS, GBIF, AquaMaps, Marine IBA e-atlas, MAPAMED and others with marine and maritime links. Through ODYSSEA’s end-user centred approach, in which the various groups of end-users and stakeholders, within and external to the Consortium, will be involved from Day 1 of the project in the design, development and operation of the platform, including identification of gaps in data collection and accessibility High priority gaps will be filled through multiple approaches that include developing a network of coastal observatories, deploying novel in-situ sensors at sea (a.o. microplastic sensors), oceanographic modelling and integrating existing mobile apps for citizen scientist networks. Applying advanced algorithms to organise, homogenise and fuse the large quantities of data in common standard type and format as well as other types of formats, the ODYSSEA platform will provide both primary data and on-demand derived data services, including forecasts, from ALL Mediterranean countries through a SINGLE PUBLIC PORTAL to various end-user groups and stakeholders. End-user requirements will drive the creation of secondary data sets which the platform will provide as new and packaged services matching the specialised information needs of users. ODYSSEA will improve accessibility to existing data as well as increase the temporal and geographic coverage of observational data in the Mediterranean.