Acoustic Condition monitoring (CM) have many advantages comparing to other traditional CM methods (using vibration, speed measurement, temperature monitoring, .etc) It is reliable, cost effective and providing remote and contact less measurement. However, since acoustic signals have to propagate through the working environment before received by sensors,the performance of acoustic CM systems can be seriously influenced by environment factors,especially when they are working in industrial environment which normally has very strong background noise and reverberation. In this book, a number of newly developed technologies were introduced to reduce this influence. A small scale acoustic array based condition monitoring system were also introduced as an example using these technologies to monitor machines' working conditions and diagnose their potential faults in practical industrial environment.