Recently, considerable interest has been expressed in the use of radar to detect underground targets both small (e.g., antipersonnel mines) and large (e.g., buried vehicles). Particular interest has been directed at airborne SAR for this purpose. Several important issues requiring study include the scattering signature of objects buried in soil media, the attenuation and scattering of radar energy in inhomogeneous soils, and the impact of clutter (and particularly the impact of surface clutter layover) on subsurface target detection and recognition. To address these issues, a radar ground penetration experiment was conducted in the desert near Yuma, AZ from June 4 to 15, 1993. In this experiment a number of large and small targets of various shapes were buried at depths up to 3 m, and data was collected using several air- and ground-based radars using both real and synthetic aperture data processing. The variety of radars available covered the range from 20 to 1500 MHz. The data collected was calibrated with sufficient accuracy to permit the measurement of in situ radar signatures, allowing the calculation of ground penetration losses. Data from this test have been analyzed to develop a phenomenological understanding of soil penetration losses and clutter backscattering, and to investigate the characteristic signatures of specific buried targets. These data are compared to laboratory soil measurements and modeling studies. This paper will describe the experiment, sensors, sample radar measurements and some of the results of the data analysis.