In this paper we review the contributions of GNSS ground-based and radio occultation receivers to the understanding and prediction of severe weather phenomena around the world. These ground- and space-based GNSS observations, which are complementary to other in-situ and remotely sensed observations, are sensitive to the temperature and water vapor content of the atmosphere, both important parameters that characterize the structure and evolution of heavy rainfall and convective storms, atmospheric rivers, tropical cyclones, and droughts and heat waves. With the first ground-based GPS observations reported in the early 1990s and the first radio occultation observations of Earth's atmosphere derived from the GPS/MET proof-of-concept mission (1995–1997), these GNSS-based observations are still relatively new contributors to the research and operational suite of technologies.