At a time when most polar cap ionospheric studies are focused on electron density enhancements caused by patches or arcs, this paper discusses an F region depletion known as the ionospheric polar hole. Statistically, polar holes tend to develop on the nightside from 2100--0600 MLT between 70¿--80¿ &Lgr;. Electron concentrations as small as 2¿102 cm-3 at altitudes near 300 km have previously been observed in polar holes during solar minimum winter conditions. Under magnetically quiet conditions, polar holes are thought to form due to slow convection of plasma across the polar cap. The electron concentration is depleted by normal recombination processes during many hours spent in the dark polar cap. In contrast with previous studies, which have tended to be statistical in nature, this paper describes the temporal and spatial development of a single polar hole. During the Geospace Environment Modelling Pilot Program of January 16, 1990, the development of the polar hole was monitored by the DMSP F8 and F9 satellites, a digital ionosonde and a 250-MHz scintillation receiver. The data reveal that the polar hole is a very dynamic phenomenon, and its location can change dramatically within several hours. This is the first study of the polar hole using ground based instruments. The observations presented here are of interest not only because they contribute to a morphological understanding of the polar hole, but also because they illustrate the influence of the polar hole on radio signals propagating in the ionospheric medium. The results are of direct relevance to applications and systems involving ionospheric and transionospheric propagation. The data set is sufficiently complete that it will provide rigorous constraints for future modeling studies and will thus contribute to a better physical understanding of the polar hole. ¿ American Geophysical Union 1993