This paper discusses a three-dimensional synthetic aperture imaging technique based on time- domain focusing of pulse-echo radar data. We describe the basic image formation process, important data processing issues, and compensation for planar variations in the media. We present a high-resolution volumetric image reconstruction of a concrete test slab and show that we are able to identify steel reinforcing bars in the image. We conclude with a brief comparison of this imaging method with a technique based on diffraction tomography.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.