This chapter presents the Gravity Recovery and Climate Experiment (GRACE) mission, its fundamental measurements, and how these measurements are used to observe changes in the Earth's surface mass distribution and, in particular, terrestrial ice mass evolution. From the beginning of the space age, observations of satellite motions have been used to compute gravity models of the Earth. Milo Wolff was the first to introduce the concept of computing the variations in the Earth's gravity field directly from observations of the changing range between two low Earth co-orbiting satellites. The GRACE mission Level 1B (L1B) data processing centers all use their own sophisticated processing systems but, for the purpose of the chapter, the author uses the GEODYN system and processing approach. The contributions to the computed inter-satellite ranging observations from the Earth's static gravity field, planetary bodies, ocean tides, solid Earth and pole tides are computed within GEODYN.