Geodesy is the science of determining the size and shape of the Earth (including its temporal variation) using measurements primarily (today) of distance, time, and gravity. Being one of the oldest sciences, with a history of more than two thousand years, the traditional measurements were mostly associated with land surveying (distance and direction measurements of landmarks and celestial objects) and gravity observations (to determine the geoid, as reference surface for heights, and the plumb direction). The age of satellites, radio science, and computer technology have completely transformed these methods and enabled geodesy to branch into many of the Earth sciences where the exquisite measurement precision has enabled the determination of ocean level variations, crustal deformation, tectonic plate motions, ice sheet heights, Earth rotation variation, and other geodynamical phenomena with unprecedented detail and accuracy. Contemporary geodesy utilizes the latest in mathematical modeling, physics research, astrometry, computer science, and statistical analysis to aid in the understanding of ocean currents, sea level rise, the world's hydrological cycles, atmospheric conditions, global climate change, post-glacial rebound, and other crustal deformation, particuarly as it relates to natural hazards, such as earthquakes, volcanoes, and flooding. In these Earth science applications, accurate coordinate reference frames, high-resolution global gravity models, and precise time keeping are of paramount and fundamental importance. Geodesy, of course, continues the tradition of forming the backbone for all national and international datums and reference systems needed to establish three-dimensional positional control of regional and global networks of terrestrial points. Several geodetic principles and techniques also have found application in studies of the Moon and other planets.