Knowledge of the ultimate sensitivity of the acoustic emission (AE) method is useful in studies of the mechanisms of plastic deformation of the structure, the formation and development of micro-, meso- and macro-cracks, as well as continuous processes, such as the outflow of liquids and gases, friction and a number of others. Analysis of literary data on assessment of limit sensitivity at identification of AE sources was carried out. It has been shown that when using standard resonant piezoelectric transducers with a frequency bandwidth of 30 ± 10 kHz, the ultimate sensitivity is the fraction of the nanometer in the displacement of the surface of the object and the unit micron of the size of the microcrack during its formation and hopping development. When testing industrial facilities, in many cases the level of external noise is significantly higher, the detection of defects decreases and amounts to a fraction of a millimeter. However, an increase in the energy and amplitude of AE signals as the defect develops in the vast majority of cases leads to the fact that when a crack reaches dimensions that begin to threaten the strength of the monitored object, AE signals are reliably detected by the equipment. Knowledge of the maximum sensitivity of the AE method makes it possible to compare it with other NDT methods by this parameter. Keywords: acoustic emission, detection capability, flaw size, detection probability, cracks, dislocations, piezoelectric transducer.