ExplosionsTerrorist devices and military ordnance typically contain high explosives. A high explosive (e.g. trinitrotoluene, gelignite, Semtex) is a compound (solid or fluid) which can undergo an intense exothermic reaction, releasing large amounts of energy very quickly. This process propagates at supersonic speeds (approximately 5000 m/s -) radially from the explosive and is termed detonation. A shock wave (blast wave) is generated in the surrounding air and resulting variations in air pressure set in motion the mass movement of air (the dynamic overpressure or blast wind). The shock wave, the dynamic overpressure and thermal energy are the underlying phenomena which contribute to blast injuries.By convention, blast injuries are classified according to the mechanism by which they are produced (Table 1). (Wang et al 1993).Explosions are associated with a high incidence of psychological sequelae in injured and uninjured survivors.A detailed discussion of the generation and propagation of blast waves and their interaction with structures is given elsewhere in this volume (Cullis). An overview is presented below.The variation in blast wave pressure with time, at a fixed point in space (the Friedlander relationship) describes the Lt Col C L Horrocks FRCS (ORL-HNS) RAMC