Radio-echo sounding (RES) constitutes the principal means by which glaciologists investigate the subsurface properties of the polar ice sheets and ice caps. Developed in the 1960s as a method for locating and mapping the subglacial interface beneath extensive regions of ice-covered terrain, thereby to constrain ice volume and morphology, it was quickly discovered that RES supplies numerous additional cryospheric parameters, including strong reflectors derived from subglacial lakes, and isochronous internal reflectors derived from burial of snow deposition events. Soon after its establishment, RES was integrated into long-range aircraft primarily to image the bed across Antarctica and Greenland (1960s/1970s). More recent airborne campaigns (1980s/1990s), while supplementing this coverage and extending to the ice caps of the High Arctic, have utilised only short-range aircraft, and were designed explicitly to support specific scientific studies, such as locating optimal sites for deep ice-coring, constraining the dimensions of subglacial lakes, or resolving internal layers for studies of ice sheet mass balance, form and flow. In parallel with these developments, ground-based (over-snow) RES equipment has also been used to investigate the englacial and subglacial conditions at a number of key locations across the polar ice sheets. This article discusses the many scientific advances which have resulted from these efforts, and offers recommendations for future developments in terms of (i) reanalysis of existing data and (ii) suggestions for future RES campaigns.