The collapse of a void within a reactive material has the potential to start local burning leading to partial reaction or run to detonation. There are three main features of the collapse that provide a means for ignition. The first is the formation of the high-speed jet and elevated velocities in the convergent flow around the wall of the cavity. This gives rise to heating in viscous materials. The second is the shock-heated region at the point of jet impact in an asymmetric collapse. The third is the compression of any gaseous or vapour content. These effects have been studied and experiments showing examples of each of these modes of heating giving rise to local reaction are described. Once ignited, a burning front may be quenched or may accelerate so that a transition to detonation may occur according to the confinement of the material.