Water quality is a key worldwide issue relevant to food production, industry, nature and recreation. Recognising its importance, Copernicus has satellite data and services to provide water quality data and information to end-users in industry, policy, monitoring agencies and science. However, water quality data production is split across three services, Copernicus Marine, Climate Change, and Land, with different methods used, while transitional waters are not supported by any service. CERTO will address these issues by undertaking R&D necessary to produce harmonised water quality data from each Copernicus service and extend support to the large communities operating in transitional waters. CERTO will focus on: methods to classify waters, using satellite observations, together with existing and new in situ data; improvements to remove the atmospheric signal, particularly problematic in near-coastal and transitional waters; and evaluating cross-cutting optical water quality Indicators, that may be used across coasts, transitional and inland waters (monitored through WFD and MSFD). The project will contribute to DANUBIUS the developing European research infrastructure in River-Sea Systems, GEO AquaWatch and Blue Planet, the Lagoons for Life initiative as well as supporting the United National Sustainable Development Goals. The main output of the project will be a prototype that can be “plugged into” the existing services, or the Copernicus DIAS, and widely used open-source software (SNAP). CERTO will also produce the evidence needed by the “entrusted entities” that run Copernicus services as to the improvements, potential to increase the user community, possible downstream services and wider impact of the prototype. CERTO will achieve it objectives by bringing uniquely together the leaders of the water quality data elements in the three Copernicus Services, 5 SMEs Climate-KIC, 4 research intensive institutes/and leaders of end-user relevant communities.