Most crystals are convex in shape and therefore can be uniquely described by vectors which are normal to the crystal faces. The magnitude of these vectors is determined by the minimum distance between an arbitrarily chosen interior point and the surfaces, extrapolated where necessary. General expressions are derived which relate the geometric quantities of a convex polyhedron such as volume and surface area to these vectors. This analysis provides a basis for a formal treatment of the evolution of crystal size/shape distributions in crystallisers and dissolvers. By way of illustration the problem of growth in a batch crystallisation process is analysed in some detail. A general proof of the cube root law, which predicts that the volume of a crystal, grown under conditions of constant supersaturation, has a cubic dependence on time is given.