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Acoustic emission (AE) is one of the most important non-destructive testing (NDT) method for materials, constructions and machines. The acoustic emission method differs from most other non-destructive testing techniques in two crucial respects. First of all, the source of the signal is always located inside the investigated material/structure. The second important thing is that the NDT methods detect geometrical discontinuities, while acoustic emission detects fault movements. According to the ASTM Guideline of Standard Terminology for Nondestructive Examinations, acoustic emission is commonly defined as the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localized sources within a material. This energy dissipation is basically due to phase transformation, crack formation and growth, friction or moving dislocations. Acoustic emission signals may be also generated by such phenomena as leaks, cavitation, solidification and liquefaction.