Summary The physical properties and propagation characteristics of atmospheric infrasound determine the techniques and instrumentation used for its measurement. The physical properties which influence the design of the measurement system for infrasound are summarized, and the techniques and instrumentation used for measurement are described. Propagation over long distances is influenced by temperature gradients and winds. Therefore a network of infrasonic stations has been established on a world-wide basis to carry on propagation researches. Investigators at the observatories have identified various geoacoustical sources of infrasound. This paper provides a description of the basic measurement system used at all of the stations in the network. We are concerned with the measurement of infrasound in the atmosphere, i.e. sound waves whose frequencies of oscillation are less than the lowest audible frequency (about 15 Hz). Of particular interest are those waves whose oscillation periods lie in the range of 1 *O-lOOO s, because such waves propagate for distances of thousands of kilometres without substantial loss of energy. Sounds at these frequencies are almost always present at measurable intensities. Those of natural origin have many causes, including volcanic explosions, earthquakes, waves on the seas, and large meteorites. Severe storms and the aurora in the northern hemisphere are often accompanied by infrasound. Man-made sources include powerful explosions and the shock waves from vehicles moving through the atmosphere at supersonic speeds, at altitudes below about 125 km. Brief preliminary descriptions of the basis for an infrasonic measurement system and for its design have already been published (Cook & Young 1962; Cook 1969). Here we review the earlier brief descriptions and provide a fuller definition of the techniques and basic instrumentation for measurement of infrasound. The passage of an infrasonic wave causes pressure oscillations as it traverses the atmosphere. For infrasound of natural origin, the amplitude p of the sound pressure is often in the range of 0.1-100dyncm-2, and infrasonic microphones are usually designed to respond to such pressures (the atmospheric pressure Po x lo6 dyn crn-'). Experience has shown that infrasonic waves observed at a station are approximately plane, propagating in a direction which is usually almost parallel to the Earth's surface. The several microphones at a station are all on the Earth's surface, and the speed of a line of constant phase for a sound wave travelling over the Earth's surface can be determined from the output of the several microphones. This speed c,, is usually