Interfaces can be considered at a variety of length scales. All interfaces except grain boundaries are dielectric interfaces. In many cases, the geometric constraints of matching two lattices must be considered, together with the misfit strains that are often present. Continuum mechanics is useful for tackling such problems. In many cases, however, the local ordering of ions must also be considered. Atomistic simulation is therefore necessary, together with the problems associated with large length scales and long time scales. We discuss a number of examples to illustrate the issues involved and the compromises between different approaches that must be made.