The Aerial Radiological Measuring System (ARMS) designed and operated by EG&G, Inc., for the Division of Biological and ion of Reactor Development, U. S. Atomic Energy Commission is described. Designed to rapidly survey large terrestrial surface areas for low-level background radiation, the ARMS has also proven extremely useful in locating lost radioactive sources of relatively low activity. The system consists of sodium iodide scintillation detectors, data processing electronics, Doppler radar positioning equipment, meteorological instruments, recording equipment, and direct readout hardware. The instrumentation, operational procedures, and data reduction techniques are described, together with their applications and sample results.