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This paper surveys the present-day description of ferroelasticity in terms of the notion of symmetry descent. Based on the work of earlier authors, a symmetry classification of phase transitions is presented. A general classification of twinning in crystals is also attempted on the basis of this scheme. Most kinds of twinning fall into two broad categories: nonferrodastic and ferrodastic. The shape-memory effect associated with martensitic transitions is discussed briefly. The interesting possibilities of this effect in the case of ferroelectric ferroelastics, for which the electric field provides an additional control parameter, are also mentioned.

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