Atmospheric retrievals using radio occultations (RO) opportunistically provided by the Global Positioning System (GPS) are expected to be available into the indefinite future, given the success of COSMIC and the strong support for GPS RO in the NRC's recent Decadal Survey. RO retrievals have the fortunate characteristic of being based on time delay measurements, whose fundamental unit-the second-is absolutely determined by calibrating against atomic clocks. Multi-decadal time series of RO retrievals are natural to consider for climate monitoring of air temperatures from the upper troposphere to the stratosphere. Stringent climate monitoring requirements of 0.1 K accuracy or better requires that systematic error sources in RO retrievals be carefully analyzed to establish SI-traceability of the temperature retrievals. Even if such calibration is conclusively established for a particular retrieval system, it remains to establish the best use of GPS RO for long-term climate monitoring and as a means for calibrating other observing systems. Factors that must be accounted for are the number and global distribution of measurements, the spatial volume sampled, and precision and accuracy as a function of altitude range. We will discuss recent results from GPS RO instruments onboard the COSMIC, CHAMP, SAC-C and GRACE satellites. Very high precision has certainly been achieved. What are the implications for accuracy versus altitude? We will present recent results comparing GPS RO retrievals to other systems such as sonde, microwave and infrared sounders, and discuss ways to exploit the absence of long-term drift from GPS RO retrievals. GPS RO should also be viewed in the context of the broader picture of climate understanding since it is well suited for comparing to climate model output, as is the focus of the CLARREO mission. Achieving full benefit from GPS RO as part of an integrated observing system suggests the importance of wide community utilization and understanding of the technique. Care must be exercised to assure that adequate numbers of measurements are available into the future.