GMES (Global Monitoring for Environment and Security) is a joint initiative of the European Commission and the European Space Agency, designed to establish a European capacity for the provision and use of operational information for Global Monitoring of Environment and Security. To support the development of the Ocean and Marine Applications component of GMES, the EC is funding the MERSEA integrated project. The system is based on the assimilation of remote sensing (altimetry, sea surface temperature, sea ice) and in situ observations (ARGO and XBT profiles) into high resolution ocean models. The project includes research and development activities on data products, ocean modelling, assimilation, nesting and downscaling, ecosytems, and seasonal forecasting. The project federates the European contribution to the GODAE. It intends to contribute to the development of integrated core ocean services in Europe. The system design includes a global, as well as four regional components (Nordic, Baltic / North sea, North East Atlantic, and Mediterranean seas) which provide ocean analyses and forecasts in real time through assimilation of in situ and satellite data. The essential variables are the temperature, salinity and velocity fields, but research efforts are underway to include bio-geochemical and ecosystem variables. The input data and forcing fields are provided by dedicated Centres. The whole system is linked through an Information Management System. The overarching objective is to provide an integrated service of global and regional ocean monitoring and forecasting to intermediate users and policy makers in support of safe and efficient offshore activities, environmental management, security, and sustainable use of marine resources. Some forty agencies, laboratories and institutions participate in the research and development effort of this four year project, which started in 2004. The project is oriented towards applications, and the delivery of products and services in support of marine safety and offshore industry, marine meteorology, wave forecasts and ship routing, oil drift fate prediction, seasonal forecasting, research, and reporting to public bodies. The system is also intended to deliver boundary conditions and data to coastal operational systems. Since Octobre 2005, the system has been upgraded to its version V1, and an intensive pre-operational test period will be run for six months.