Subsurface sensing is a problem of longstanding importance to the remote sensing community. This encompasses such problems as sensing for minerals, oil, and characterization of general subsurface geophysical properties. Many of these applications involve sensing at significant depths with the attendant instrumentation and geophysical complications. There are several subsurface sensing problems of interest to military applications of importance during and after a conflict. The postconflict environmental cleanup constitutes an important humanitarian mission. These problems include sensing buried land mines, unexploded ordnance (UXO), and improvised explosive devices (IEDs). These are typically near-surface sensing problems that require careful attention to phenomena manifested at the air-soil interface. This includes consideration of rough-surface phenomenon, near-surface clutter, and biomass.
 In this special issue we primarily focus on sensing of land mines, although the closely related problem of UXO sensing is also considered. From a tactical standpoint, land mines and UXO are quite different: land mines are designed to injure and kill people, while a UXO represents ordnance that did not operate as planned (it did not explode on impact). While the UXO did not operate as planned, it subsequently presents a significant danger if carelessly disturbed. In addition to these differences, many land mines are small and of low metal content, while UXO are typically larger and made of metal. These differences make a significant difference in sensor selection and associated signal processing.
 The primary sensors employed for sensing land mines are radar, seismic, electromagnetic induction (EMI), and infrared systems. Magnetometers and EMI are the primary sensors applied for UXOs. Other newer experimental systems are based on chemical sensors, motivated by the goal of replicating the excellent sensing performance of dogs and other mammals. In this special issue we focus on sensors of interest to the Radio Science community and therefore the principal focus is on radar, seismic, and EMI sensors. The papers presented here cover sensor physics as well as associated signal processing. These papers provide a good representation of the state of the art.
 The Guest Editors thank the Radio Science Editor for inviting us to put this special issue together. It is hoped that these papers will provide a good measure of the current capabilities and challenges in the land mine and UXO problems, pointing the way for new improvements as the research community pushes toward solutions for these important military and humanitarian problems.