Early in 1984, NASA asked the Space Science Board to undertake a study to determine the principal scientific issues that the discipline of space science would face during the period from about 1995 to 2015. This request was made partly because NASA expected the Space Station to become available at the beginning of this period, and partly because the missions needed to implement research strategies previously developed by the various committees of the board should have been launched or their development under way by that time. A two-year study was called for. To carry out the study the board put together task groups in earth sciences, planetary and lunar exploration, solar and space physics, astronomy and astrophysics, fundamental physics and chemistry (relativistic gravitation and microgravity sciences), and life sciences. The findings of the study are published in seven volumes: six task group reports, of which this volume is one, and an overview report of the steering group. This report outlines a unified program for studying the Earth, from its deep interior to its fluid envelopes. It proposes a system of measuring devices involving both space-based and in situ observations that can accommodate simultaneously a large range of scientific needs.