More and more satellites apply the GPS (Global Positioning System) radio occultation (RO) technique for precise sound-ing of the Earth's atmosphere and ionosphere on a global scale. A milestone for the recognition of this quite new remote sensing method was reached, when ECMWF and MetOffice started operational assimilation of GPS occultation data to improve global weather forecasts in 2006. Currently the major part of the operational available RO data is provided by the six satellite U.S.-Taiwan constellation FORMOSAT-3/COSMIC (FORMOsa SATellite mission-3/Constellation Ob-serving System for Meteorology, Ionosphere and Climate). Data from the European operational Metop satellite (GRAS -GNSS Receiver for Atmospheric Sounding, launched 2006) are expected to be available in the near future. However, besides these dedicated RO missions, the operation of several GPS RO receivers is based on a mission-of-opportunity (MoO) concept. These missions, e.g., the German CHAMP, brought significant progress for the GPS RO technique and its break through. We briefly review the status of the current MoO missions, which provide GPS RO measurements, and summarize related science results.