The electronic structure is considered in the vicinity of a crystalline point defect. The atoms of the defect and its nearby surroundings are treated as a quantum molecular cluster. A general theory is developed in the context of a Hartree-Fock computational scheme which takes account of atom-atom overlap between the cluster and the rest of the crystal in quite a rigorous way. This is referred to as an embedding procedure. In ionic materials, although overlap effects may be weak and require less rigorous treatment, long-range electronic perturbation in the form of dielectric polarization, induced by charged defects, is not covered by the quantum-mechanical analysis. Therefore, a model which embeds, the quantum cluster in an infinite, classical shell-model crystal is described, along with its implementation in the ICECAP program. Approximate methods for quantum-mechanical embedding-overlap features of the cluster are discussed for this case of ionic crystals. Calculated results are then presented for a wide variety of defect problems, including different host materials and defect types, properties and processes. Generally good agreement with experimental results marks this methodology as suitable for predictive use in the simulation of defects in ionic materials.