Acoustic emission is a technique that is being used increasingly in the field of structural integrity assessment using fracture mechanics. The dynamic behaviour of defects is extremely important as a small defect that is growing may well be more significant than a larger stable defect. Acoustic emission is the method used to investigate the behaviour of defects under stress. When a structure is subjected to an external stimulus (change in pressure, load, or temperature), localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors. With the right equipment and setup, motions on the order of picometers can be identified. Sources of AE vary from natural events like earthquakes and rock bursts to the initiation and growth of cracks, slip and dislocation movements, melting, twinning, and phase transformations in metals. In composites, matrix cracking and fibre breakage and debonding contribute to acoustic emissions. AEs have also been measured and recorded in polymers, wood, and concrete, among other materials. This book entitled Acoustic Emission - Research and Applications shows that the AE method is constantly and intensively emerging and broadly applied in: on-line monitoring of civil-engineering structures, fatigue cracks detection and location in pressure vessels and pipelines, damage assessment in fibre-reinforced polymer-matrix composites, monitoring welding applications and corrosion processes, bearing condition diagnostics, partial discharge sources detection and location in power transformers and generators, monitoring the drying process of materials, and in otoacoustic emission analysis.