Diffuse X-ray scattering is a rich (virtually untapped) source of local structural information over and above that obtained by conventional crystal structure determination (crystallography). The main aim in the book is to show how computer simulation of a model crystal provides a general method by which diffuse scattering of all kinds and from all types of materials can be interpreted and analysed. Part I of the book gives a description of the experimental methods used to obtain diffuse scattering data. Part II describes a number of simple stochastic models of disorder, which allows various concepts to be established and enables simple examples to be generated to illustrate key principles. Part III describes example studies of a wide variety of real materials. These examples not only document the development of computer simulation methods for investigating and analysing disorder problems but also provide a resource for helping future researchers recognise the kinds of effects which can occur and for pointing the way to tackling new problems which are encountered.