We describe a step-frequency radar (SFR) imaging system that is suitable for microwave nondestructive evaluation (NDE) applications. The system includes a computer-automated microwave measurement apparatus, calibration software, and nonlinear inverse scattering imaging algorithms. Through the use of an inverse Fourier transform, the raw SFR data is transformed into a synthetic time-domain pulse. A calibration procedure involving the use of calibration targets may then be used to remove pulse distortions due to the effective aperture and dispersive nature of the antennas and measurement system. Nonlinear inverse scattering algorithms described here are then applied to the corrected data to produce quantitative spatial images of various electromagnetic parameters. Reconstructions of metallic and dielectric scattering objects including metallic rods, glass rods and plastic PVC pipes from real measurement data collected in our laboratory are shown.