(), it was decided that infrasound was one of four techniques that would be used to verify it. Infrasound is very low-frequency sound that is inaudible to humans. The term 'infrasound' is analogous to 'infrared', the part of the visible light electromagnetic spectrum with lower frequencies than the red part. Infrasound monitoring is directed towards the atmosphere where the sounds of nuclear explosions propagate. The other technologies used are seismology for the verification of nuclear explosions in the earth; hydroacoustics for ocean basins; and the measurement of airborne radionuclides to detect atmospheric fallout. Three media and four techniques might at first sight seem to be overkill. However, for the atmosphere there is no single earth-based technique that is able to locate and identify nuclear explosions. Satellite technology was considered at the time of the negotiations, but was judged to be too expensive. The measurement of radionuclides can provide the 'smoking gun' that identifies a nuclear explosion but is not well suited to locating the event. Infrasound complements the radio-nuclide measurements because it enables analysts to locate the events faster in time and space. This chapter considers various aspects of infrasound as a tool for verification. As a young and relatively unknown technique compared with the other technologies, it needs some technical explanation if we are to understand its role and contribution to the verification system. While the infrasound technique could be considered to be the equivalent of seismology for the atmosphere, in fact, the atmosphere has a complicated dynamic structure that is unlike the solid earth. This adds to the complexity of the technique.