The Global Positioning System (GPS) has become a widely used tool in geodetic studies of Earth. We review the principles of the applications of GPS to geodetic problems and discuss its applications to problems of global and regional geodesy. A global network of dual-frequency GPS receivers continuously tracks the GPS satellites and the data are distributed to globally accessible on-line archives with delays from a few hours to a few days. Analysis of these data occurs within a few days and the results are made available in the form of satellite ephemerides accurate to a few centimeters and receiver position estimates accurate to a few millimeters for weekly averaged positions. The collection and analysis of these data occurs under the auspices of the International GPS Service for Geodynamics (IGS) which maintains Internet-accessible archives in the United States and Europe. The data and analyzed results are used to study problems of global geodynamics with direct application to global plate tectonics and postglacial rebound and to studies of the the excitation of variations in Earth's rotation. When the global data are combined with data collected in a local region they can be used to study the deformation processes in that region. In the long run, analysis of these regional data will lend to improved dynamical models and to a better understanding of earthquakes and other deformation processes.