A time-domain approach for dielectric characterization using a stripline geometry is presented. The technique uses both Time Domain Reflectometry (TDR) and Time Domain Transmission (TDT) measurements for determining an optimum frequency-dependent lossy transmission line model for the stripline under test. The optimization is done in the time domain by comparing the experimental TDR and TDT response waveforms with the simulated ones using a nonlinear least squares fit. The material properties such as the complex permittivity of the dielectric material are determined from the optimum lossy transmission line model. The paper presents both simulated and experimental results.