Polar orbiting or geostationnary earth observation missions are vital for operational applications (meteorology, environment monitoring, ocean circulation forecast,...) and long-term survey of the climate and the ocean. They provide global observations on a regular and consistent basis for numerous geophysical or biogeochemical parameters over the ocean, including surface winds, waves, heat content, sea level, sea surface temperature, sea ice, air/sea fluxes,...The retrieval of this information is performed through a wide range of onboard instruments including active and passive sensors operating in various wave length. This section provides an overview of the various missions and their benefits to ocean knowledge.