The use of ground penetrating radar (GPR) is a valuable method for remotely detecting the presence and movement of DNAPLs within an aquifer. However, the strength and limitations of the procedure, as applied to a particular site, are important to understand. GPR detects the reflections of radar energy returned from the subsurface produced by changes in the electromagnetic properties of the materials encountered. GPR signals from aquifers without DNAPLs will produce reflections created by subtle changes in porosity because the electromagnetic properties of sand and water are quite different. In addition, mineralogical clay can also produce reflections due to the higher conductivities of the clay. These clays can also reduce the depth of penetration of the GPR signal. Since most DNAPLs have very different electromagnetic properties from water, any DNAPL replacement of water will produce reflections of the GPR signal. In addition, the travel time of the GPR signal through the subsurface becomes faster as the amount of DNAPL increases. In a heterogeneous aquifer, DNAPLs can most easily be detected with the comparison of a pre-DNAPL scan versus a post-DNAPL scan. However, at most sites, a pre-DNAPL GPR scan is not available. In this case, the amplitude and the return time of GPR reflections are the key factors in determining the location and quantity of DNAPL. This requires some background knowledge of the aquifer in terms of the magnitude of aquifer porosity and the character of clay zones. The substantial difference in permittivity of DNAPL facilitates identifying likely zones of high DNAPL saturation even when pre-spill surveys have not been conducted. The existence of a "target" layer is also a key factor in determining the quantity of DNAPL. A "target" layer, such as a clay lens or the base of the aquifer, provides a known depth for quantifying the faster travel time of the GPR signal through a DNAPL zone. Unaffected areas of the aquifer can be surveyed to gather background knowledge of the amplitudes of signals to be expected due to porosity and mineralogy variation alone, and to calibrate to a target layer. Once GPR has identified DNAPL zones, additional GPR data can be collected to track the progress of any remedial efforts.