With the availability of approximately 4000 radio occultation soundings per day within three hours of observation, COSMIC has the potential to contribute significantly to global and regional weather analysis and prediction. However, the basic radio occultation measurements (phase delays) are very different from traditional meteorological measurements (i.e., temperature, water vapor), and to effectively assimilate them into weather prediction models is a challenging task. Over the past five years, considerable progress has been made in the development of an effective strategy for the assimilation of GPS radio occultation data. In this paper, we (1) review the measurement and data reduction procedures, (2) discuss the error characteristics of the GPS radio occultation data, (3) discuss the various strategies for data assimilation, (4) review results form recent data assimilation research. and (5) provide suggestions for future research.