When a ferroelastic crystal is heated we usually find that the ferroelastic effect disappears at a well defined temperature. At this temperature a structural phase transition between a ferroelastic and a paraelastic phase takes place with the main feature being that a ferroelastic hysterisis exists in one phase but not the other. A possible transition mechanism is that the thermal excitation of domain switching occurs in such a way that the crystal maintains the crystal structure of the ferroelastic phase on a local scale even in the paraelastic phase. The domain boundaries between the different ferroelastic domains would be highly mobile and thus no macroscopic domain switching is possible; the crystal is macroscopically paraelastic. This transition behaviour has indeed been observed in very few ferroelastic materials, such as high purity lead phosphate, and we shall comment on the significance of mobile domain boundaries in Chapter 8. In the vast majority of all ferroelastic crystals, however, one finds that the physical mechanism of the transition between the paraelastic and ferroelastic phase is only indirectly correlated with the ferroelastic effect itself. Looking at it in a different way we may ask ourselves what makes a crystal ferroelastic.