The atmospheric limb sounding technique making use of radio signals transmitted by the Global Position System (GPS) has emerged as a promising approach for global atmospheric observations. By placing a GPS occultation receiver onboard a low Earth orbiting (LEO) satellite, we can measure phase and amplitude of the microwave signals emitted from GPS. These signals are inverted to obtain profiles of signal bending as a function of height, making use of the precise knowledge of the loci and velocities of the GPS and LEO satellites. Atmospheric refractivity profiles, which provide information about the electron density in the ionosphere and temperature and water vapor in the stratosphere and troposphere, can be retrieved from the bending angle profiles. As demonstrated by the proof-of-concept GPS Meteorology (GPS/MET) experiment and more recently by the CHAMP and SAC-C missions, the GPS radio occultation (RO) sounding data are of high accuracy and high vertical resolution. Moreover, the GPS RO soundings are not affected by clouds or precipitation, require no first guess, and are independent of radiosonde or other calibrations. The GPS RO measurements are not subject to instrument drifts or satellite-to-satellite biases. These attributes make GPS RO soundings uniquely suitable for long-term climate monitoring. In late 2005, the joint U.S.-Taiwan COSMIC mission will be launched and is expected to collect up to 3,000 radio occultation soundings per day. The main purpose of the upcoming COSMIC mission is to demonstrate the value of GPS RO data for operational weather forecasting and climate monitoring. This paper will review recent analysis of GPS RO data that are relevant to global change and climate variations. In particular, we will examine the tropopause climatology over the tropics and polar regions based on the GPS RO data collected from the past GPS RO missions. We will also assess the quality of global reanalyses (which are important data sets for global change studies) using the GPS RO data. This paper will highlight the usefulness of GPS RO data in climate monitoring.