Future Earth observation missions developed by the European Space Agency will be either of pre-operational nature ('Earth Watch') or will be focused on specific research topics ('Earth Explorer'). Nine scientific candidates have been selected for the Earth Explorer missions: atmospheric chemistry, atmospheric dynamics, atmospheric profiling. Earth radiation, gravity field and steady-state ocean circulation, land surface processes and interactions, magnetometry, precipitation, topography. For each of these candidates, a space mission has been defined in cooperation with Earth scientists and its implementation has been studied at pre-phase A level. The pre-phase A activities dealt with the preliminary configuration of the space and ground segments, the assessment of the requirements on the spacecraft and the preliminary study of the instruments, including breadboarding of critical technologies. The results of the scientific and technical definition studies have been presented to the scientific community and the merits of the various missions have been independently assessed.