The application of radar (or radio-echo sounding) in cryospheric sciences started more than 50 years ago. By now "radioglaciology" has become a standard method utilizing highly sophisticated measuring systems deployed on the ground or airborne, sending an electromagnetic waveform in the radio-frequency range (1-1000 MHz) into the ice and receiving the returned echos. The approach is highly effective for the Investigation of glaciers and ice sheets, as the physical properties allow high resolution and deep penetration, all the way down to the base. Applications include determination of ice thickness (determining flux geometry or undulations), bed conditions (subglacial lakes, grounding lines), internal layer architecture (reconstruction of dynamics and accumulation or basal melting rate, ice core synchronisation), density distribution (firn-ice transition, porosity), liquid water content (hydraulics, mass balance), detection of polythermal boundaries (cold-temperate transition surface, constraints for geothermal heat flux estimates) or internal conduits (hydraulics, dynamics) and crevasses (ice shelf stability, logistics) as well as other inclusion (sediments, boulders, airplanes, past stations). Numerous of the products are required as boundary conditions for numerical models as well as constraints for calibration and validation, also for satellite remote sensing. Current research frontiers include the full mapping of the Antarctic ice sheet (e.g. SCAR action groups AntArchitecture or RINGS), deployment on drones, further pushing the technical capabilities to increase high-resolution (<0.5 m) and deep penetration (>5 km) at the same time and eventually also allow the deployment on Earth-orbiting satellites, already realized for Mars.