In the last years many hydrotests, cyclic fatigue tests and thermal shock tests have been carried out on reactor pressure vessels, which exhibited crack-like defects, with the aim to determine the conditions of detectability of growing and nongrowing cracks by acoustic emission (AE). The AE generated during these experiments has been recorded and analysed. Techniques for evaluation of AE-data have been developed which allowed the separation of AE-events from crack growth and crack surface friction by using the distributions of AE signal parameters, especially the rise time. Crack growth can be detected with high sensitivity at thermal shock and fatigue tests whereas detectability of crack surface friction increases with the amount of compressive stresses between the crack surfaces. At hydrotest only a low level AE is produced by nongrowing cracks due to the lack of compressive stresses.