It is clear that one of the weakest points of current theories of point defects is in the treatment of the electronic structure of the host lattice. One of the advantages of cluster calculations is that the defect and lattice can be treated together. Great care is needed to incorporate the electrostatic field of the lattice outside the cluster correctly. Polarisation and distortion can be handled by cluster models where they are of short range, that is, for neutral defects. For charged defects hybrid models similar to those used by Wood and Opik(7,8) will have to be developed. The final method may well involve three regions: an innermost region in which the electronic structure is calculated in detail and self-consistently; a second region with model ions, interacting with each other through potentials of Born-Mayer type and with the inner region through pseudopotentials; and an outer continuum. The distortion of the whole would be controlled by some efficient algorithm similar to that used in the lattice simulation methods discussed in Chapter (1).