the techniques by which defects solids are detected and characterized, and which form the subject matter of this book, have necessitated constant revision of our concepts of perfect and defective solids. It is, for example, no longer adequate to confine our attention to isolated point defects without also considering their aggregation to form more complex defect centres and eventually even separate phases, their relationship with non-stoichiometry and their interactions with each other and with the linear and planar defects with which they co-exist. Similarly, the precise structural information now available for many materials reveals that non-stoichiometric phases may exist as modulated structures in which the composition may vary in a continuous fashion to yield commensurate, semi-commensurate and incommensur-ate structures. The presence of shear planes also changes the composit-ion of non-stoichiometric phases.