This volume is the result of the dedicated effort undertaken by an international group of scientists and administrators, who have contemplated the challenge of the future of space-based earth science for the next decade. Recognizing the need for defining new milestones both in science and technology, they have developed a detailed report of what could be achieved and what challenges remain after twenty fertile years of space exploration. The reader will find a wealth of information about the role of space geodesy in the Earth Sciences of the 1990's.