The performance of armour materials depends upon the deformation mechanisms operating during the penetration process. The critical mechanisms determining the behaviour of armour ceramics have not been isolated using traditional ballistics. It has recently become possible to measure strength histories in materials under shock. The data gained for the failed strength of the armour are shown to relate directly to the penetration measured. Furthermore, it has been demonstrated in one-dimensional strain that the material can be loaded and recovered for post-mortem examination. Failure is by microfracture, which is a function of the defects and then cracking activated by plasticity mechanisms within the grains and failure at grain boundaries in the amorphous intergranular phase. Thus, it appears that the shock induced plastic yielding of grains at the impact face that determines the later time penetration through the tile. © 2010 British Crown Copyright MOD/2010.