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The primary objective of this study was to explore the potential use of electromagnetic characteristics of soils using the Time Domain Reflectometry (TDR) technique to identify physical properties of soils. Three fundamental studies in this exploration are the frequencydependent electromagnetic properties of soils, the wave propagation in a TDR system, and the inverse analyses of TDR waveforms. The three-phase model for soils was extended to a four-phase model so as to account for the interface effect and hence the soil fineness. The physical parameters of the four-phase model are related to the frequency-dependent dielectric permitivity of the soil through a semiempirical volumetric mixing model. A spectral analysis method was developed to simulate wave propagation in the TDR system in a realistic way. Inverse analysis based on the material model and the wave propagation model was formulated using Bayesian statistics to calibrate the TDR system and to infer material properties from the measured TDR waveform. A TDR probe system was used to measure the dielectric properties of soils in a compaction mold and in the field. An experimental program was carried out to make TDR measurements of 5 different soils with different water contents and densities. The result of the inverse analysis matched the TDR waveform very well and determined the dielectric spectrum of soils. With the ability to measure the dielectric spectrum of soils, the apparent dielectric constant at a particular frequency can be calculated. Results showed that the apparent dielectric constant at 1 GHz gave much better correlation with soil water content and density than the apparent dielectric constant obtained by measuring the time between reflections from the soil surface and the one from the end of the probe. Additional work was done to assess the effects of temperature of the soil on measured dielectric constant and it showed that temperature effects were relatively small and could be accounted for in a systematic way. The use of the method for soils having large particle sizes was examined and it was found that the method was applicable for particle sizes passing the 19 mm (3/4-in.) sieve as long as the percentage of particles in the larger size range was limited. Finally, the method was examined for use with stabilizing additives for soil. While test results were obtained modified soils, the measured apparent dielectric constant changed with time after compaction whereas the water content by oven drying did not change appreciably. It is conjectured that changes in apparent dielectric constant are sensitive to the hydration caused by these additives. Thus, the use of the TDR method can be a tool to monitor changes in soils containing additives. Detailed study of this phenomenon is continuing.