In earlier papers we have described the development of photographic and pressure-measuring techniques to study the detailed behaviour of thin layers of explosive when impacted. Ignition, when it occurs, invariably does so at local sites where 'hot spots' have been generated. The present paper gives the evidence for ignition by a variety of mechanisms including adiabatic shear of the explosive, adiabatic heating of trapped gas spaces, viscous flow, friction, fracture or shear of added particles, and triboluminescent discharge. Some of these mechanisms have been suggested earlier though others have resulted from our recent work. The value of the new photographic evidence is that it allows us to assess the viability of each mechanism for different explosives and impact conditions.