We review the development and status of GPS geodetic methods for high-precision global time and frequency comparisons. A comprehensive view is taken, including hardware effects in the transmitting satellites and tracking receiver stations, data analysis and interpretation, and comparisons with independent results. Other GPS techniques rely on single-frequency data and/or assume cancellation of most systematic errors using differences between simultaneous observations. By applying the full observation modelling of modern geodesy to dual-frequency observations of GPS carrier phase and pseudorange, the precision of timing comparisons can be improved from the level of several nanoseconds to near 100 ps. For an averaging interval of one day, we infer a limiting Allan deviation of about 1.4 × 10⁻¹⁵ for the GPS geodetic technique. The accuracy of time comparisons is set by the ability to calibrate the absolute instrumental delays through the GPS receiver and antenna chain, currently about 3 ns. Geodetic clock measurements are available for most of the major timing laboratories, as well as for many other tracking stations and the satellites, via the routine products of the International GPS Service.