The concept of atmospheric profiling by GPS occultations was first demonstrated with the GPS/MET experiment in 1995. The concept is derived from planetary occultation experiments where measurements of signal time delay of a spacecraft occulted behind a planet as viewed from Earth is used to infer properties of the planet’s atmosphere. Applied to Earth, the transmitter is a GPS satellite and the receiver is placed on a low-Earth orbit (LEO). The fundamental measurement is the time delay of the transmitted GPS signal as the satellite sets or rises behind Earth’s atmosphere. Precise time delay measurements are converted into atmospheric Doppler shift and bending, from which high vertical resolution profiles of atmospheric refractivity, density, temperature, pressure and water vapor can be inferred. The GPS occultation concept was first demonstrated with the GPS/MET experiment in 1995. Several missions featuring GPS occultations followed including the most recent German CHAMP (CHAllenging Minisatellite Payload) and Argentine SAC-C (Satelite de Aplicaciones Cientificas-C) missions which carry a new generation of GPS receivers with several enhanced features such as improved tracking in the lower troposphere. Data from these missions are analyzed to examine their performance in the lower troposphere for sensing water vapor. Their ability to sense the planetary boundary layer (PBL) height is particularly emphasized. The very sudden change in refractivity experienced by a GPS-LEO occulted signal descending from the lower free troposphere into the PBL causes the signal to manifest several distinct features including (1) large bending which results in strong signal defocusing immediately above the PBL top, (2) the temporary disappearance of the signal (caused by atmospheric ducting) as the occulted signal descends immediately below the PBL top, (3) the reappearance of the signal after it has descended sufficiently into the PBL. These features can be utilized to determine the PBL height with a vertical resolution exceeding 100m near the occultation tangent point. The approach for detecting the PBL from GPS occultations is discussed and Global climatologies of PBL height derived from CHAMP and SAC-C are presented.