The planetary boundary layer (PBL) height is an important parameter that describes much of the diurnal, synoptic, and climatological processes associated with the PBL in a given region, including its cloud characterization and connections between the surface and free troposphere. However, the global climatology of the PBL is poorly established due to a lack of observational data, particularly over the oceans. With high vertical resolution (about 200 m) and ability to penetrate cloud as well as global and diurnal cycle coverages, the GPS radio occultation (RO) measurements provide an unique opportunity for the remote sensing of the Earth's PBL. Early results obtained using various retrieval products from GPS RO have demonstrated great promise. In this talk, we will describe two algorithms based on the specific humidity profiles derived from GPS RO: one based on local vertical gradients and the other based on the bulk moisture content. The strengths and limitations of these approaches will be discussed. Bias and uncertainty in PBL height determination due to measurement and retrieval errors are estimated. Comparisons of the derived PBL heights with other measurements as well as models will be presented.