Lava tubes are of interest because they dramatically alter the hazard assessment of volcanos in several different ways and because they have been and are potential places to live. Ground penetrating radar has been used to locate and characterize lava tubes since 1978 in Hawaii. GPR surveys have been done over hot lava tubes where the dielectric properties modeled from the radar data give an indication of temperature and the imaging provides information about geometry. GPR data have also been acquired over, inside, around, and through cold lava tubes, and between lava tubes and the surface. Such data yield information about location, size and shape of lava tubes as well as number, distribution and occurrence. Over hot tubes containing flowing lava, commercial radar systems require modification to survive the heat. Case histories from Hawaii are illustrated with data acquisition, display, processing, and modeling.