Building on its heritage in planetary science, remote sensing of the Earth's at- sphere and ionosphere with occultation methods has undergone remarkable dev- opments since the rst GPS/Met 'proof of concept' mission in 1995. Signals of Global Navigation Satellite Systems (GNSS) satellites are exploited by radio occ- tation while natural signal sources are used in solar, lunar, and stellar occultations. A range of atmospheric variables is provided reaching from fundamental atmospheric parameters such as density, pressure, and temperature to water vapor, ozone, and othertracegasspecies. Theutilityforatmosphereandclimatearisesfromtheunique properties of self-calibration, high accuracy and vertical resolution, global coverage, and (if using radio signals) all-weather capability. Occultations have become a va- able data source for atmospheric physics and chemistry, operational meteorology, climate research as well as for space weather and planetary science. The 3rd International Workshop on Occultations for Probing Atmosphere and Climate (OPAC-3) was held September 17-21, 2007, in Graz, Austria. OPAC-3 aimed at providing a casual forum and stimulating atmosphere for scienti c disc- sion, co-operation initiatives, and mutual learning and support amongst members of alldifferentoccultationcommunities. Theworkshopwasattendedby40participants from 14 different countries who actively contributed to a scienti c programme of high quality and to an excellent workshop atmosphere. The programme included 6 invited keynote presentations and 16 invited pres- tations, complemented by about 20 contributed ones including 8 posters.