The primary objective of this study is 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 frequency-dependent electromagnetic properties of soils, the wave propagation in a TDR system, and the inverse analyses of TDR waveforms. The three-phase model for soils has been 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 permittivity of the soil through a semi-empirical 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 designed 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 traditional method.