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This paper reviews current status of acoustic emission signal detection methods, theoretical analysis of acoustic emission sources and acoustic emission behavior of materials arising from plastic deformation and fracture. Recent developments in quantitative signal detection and transducer characterization are considered. Several theories of acoustic emission sources are summarized and one based on dislocation theory by Malen and Bolin is extended to provide a relation between the inelastic displacement at a source to the peak voltage output of a resonant transducer. Models of acoustic emission from plastic deformation are presented. A new model for continuous acoustic emission is developed on the basis of the modified Malen-Bolin theory. Predictions of the model are discussed in conjunction with recent studies on acoustic emission behavior as a function of test temperature and of heat treatment. Finally, acoustic emission due to cracking is evaluated, especially in relation to amplitude distribution analysis of burst-type emission. (Kursetz)