This document specifies the general principles required for leak detection by acoustic emission testing (AT). It is addressed to the application of the methodology on structures and components, where a leak flow as a result of pressure differences appears and generates acoustic emission (AE). It describes phenomena of the AE generation and influence of the nature of fluids, shape of the gap, wave propagation and environment. The different application techniques, instrumentation and presentation of AE results are discussed. Also included are guidelines for the preparation of application documents which describe specific requirements for the application of the acoustic emission testing. Annex A gives procedures for some leak-testing applications.