The Sentinel-1, -2, and -3 satellite missions can meet various observational needs for spatially explicit physical, biogeophysical, and biological variables of the ocean, cryosphere, and land research activities. The currently known observational requirements were extracted from documents produced by major international scientific projects and programs. The summarized observational needs were then cross-referenced with the capabilities of the planned sensors aboard of the first three Sentinels. A comparative analysis, also incorporating scientific challenges of the ESA Living Planet Programme and the Essential Climate Variables (ECVs), resulted in a preliminary scientific priority assessment of the reviewed environmental variables. Results of these activities, discussed and consolidated in March 2011 at the Sentinels for Science (SEN4SCI) scientific workshop, demonstrate the high potential of the Sentinel-1, -2, and -3 missions for systematic, long-term observations of the Earth system.