Water quality variables can be derived from Earth observation (EO) data to provide essential ocean variables. They include Sea-surface temperature (SST), Sea-surface salinity (SSS) and Air-Sea Fluxes. SST controls the atmospheric response to the ocean at both weather and climate time scales. The spatial patterns of SST reveal the structure of the underlying ocean dynamics, such as, ocean fronts, eddies, coastal upwelling and exchanges between the coastal shelf and open ocean. SSS observations contribute to monitoring the global water cycle (evaporation, precipitation and glacier and river runoff). Water quality variables can be derived from EO data by using ocean colour products from optical sensors and relating them to ground truth information from in situ sensor networks.