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A brief résumé of basic magnetism is followed by a consideration of the magnetization process in the absence of applied mechanical stress, in which the importance of internal stresses and inclusions is emphasized and which leads to the introduction of Néel's disperse field theory. The Rayleigh laws are interpreted in terms of "Preisach loops." Early work on the influence of stress on the magnetization process is discussed, leading on to a detailed consideration of this phenomenon in terms of three mechanisms which, for simplicity, are taken to be essentially distinct. These are, first, the alteration of the pressure on 90° domain walls by the application of stress; second, the alteration in the opposition term, which must clearly be in balance with this pressure for domain-wall equilibrium, and, finally, large-scale alterations in the overall domain structure. Existing experimental observations, on both magnetically soft and hard materials, are interpreted in terms of these three mechanisms.