Three important parameters that need to be quantified for many permafrost studies are the location of ice in the ground, the position of thermal interfaces, and spatial variations of the water content in the active layer. The data from over 100 investigations in permafrost regions demonstrate that ground‐penetrating radar (GPR) offers an effective way to measure these parameters at a scale appropriate for many process and geotechnical studies. Horizontal to gently‐dipping interfaces between unfrozen and frozen subsurface zones (such as at the base of the active layer or a suprapermafrost talik) were repeatedly detected by GPR and indicated by strong, laterally‐coherent reflections. Coherent reflections are not generated by steeply dipping thermal interfaces (greater than 45°). However, the transition from frozen to unfrozen ground can frequently be located from the radar‐stratigraphic signatures of the two units. The radar‐stratigraphic signature of excess ice in the subsurface is determined by the size of the body. Ice lenses that are smaller than the resolution of the GPR system frequently can be detected and are represented by chaotic or hyperbolic reflections, while the size of larger ice units can be resolved and is defined by distinct laterally‐coherent reflection patterns. This enables the delineation of the vertical and lateral extent of massive ice bodies, and their structural setting. By making precise measurements of the direct ground wave velocity, the water content in the near‐surface can be determined for uniform soils. It is demonstrated that by collecting a grid of GPR data the lateral variations in active‐layer water content can then be estimated. Copyright © 2003 John Wiley & Sons, Ltd.