Four examples of acoustic emission (AE) field tests are described. In the first, standard AE source location techniques were applied to the detection of defects in insulated ethylene piping made of aluminum. Several defects were located and verified through visual and dye penetrant inspections. In the second test, approximately 7000 heat exchanger tubes were monitored for AE response during the manufacturers proof test. Monitoring with AE typically resulted in a one percent rejection rate including a few “leakers” which probably would not have been detected without AE. The initial inservice reliability of heat exchangers with tubes qualified by AE was much greater than for typical heat exchangers not monitored by AE. The third example relates results from a 50 m tall petrochemical tower, demonstrating the ability of AE to locate defects even in the presence of flow noise, tray noise, etc. In the fourth example, an explosive containment cell was monitored during hydrostatic proof test both before and after explosive experiments were carried out inside the vessel. Internal damage to the parent metal as a result of the experiments was easily detectable during the second proof test.