The methodsdescribed in this section enable us to perform accurate and reliable calculations of the properties of perfect and defective lattices where good potential models of the pair-wise type may be derived. The limitation of the type of potential model which may be effectively used in the calculations has tended to confine their application to the study of ionic and semi-ionic materials; the types of model appropriate for covalent and metallic systems are discussed in Chapters (11) and (19). Additional restrictions follow from the omission of any direct representation of quantum mechanical effects which excludes the applications of the methods to metals and semi conductors. (Treatments of defects in these systems are discussed in Chapters (13) and (14). However, in spite of these limitations, the methods we have described in this chapter have had possibly the greatest degree of quantitative success of any similar procedure in solid state studies - a feature which should become apparent in later chapters.