The aim of this book is to provide state-of-the-art information regarding the concept of Acoustic Emission (AE). AE is a significant and non-harmful testing procedure for substances, constructions and machines. It is defined as the transient elastic energy that is spontaneously released when substances undergo deformation, fracture or both. It presents significant applications of AE procedure like the evaluation of machinery and civil structures condition, fatigue and fracture substances research, detection of substance defects and deformities, monitoring of stress and ageing in substances, chemical reactions and phase transitions research, and earthquake prediction.