The use of AE by maintenance personnel for monitoring the condition of rotating machinery on the industrial shop floor is now well established and provides both a quick and effective assessment. Despite early resistance, especially by those accustomed to vibration based monitoring, it now enjoys a widespread acceptance. The development of signal processing routines and instrumentation specifically for the condition monitoring role has been a major factor in this achievement. Experience has shown that as a portable instrument AE can be very quickly applied and give instant indications of machine condition with high sensitivity to fault conditions. Appropriately pre-processed AE signals are particularly useful for on-line monitoring since the fault indications are in general less affected by changes in operating conditions than vibration based techniques as well as being far simpler to interpret. This is especially important where many machines are being simultaneously monitored. This paper discusses the accompanying developments and presents illustrative application examples.