Acoustic emission is a phenomenon which happens due to creating and propagation of an elastic wave in ultrasonic range. Although acoustic emission is an ancient technique, it is not widely used in industries. This technique can effectively help us for early detection of faults specially in roller element bearings. Acoustic emission wave can be evaluated quantitatively or qualitatively. For quantitative evaluation of an acoustic wave, many of parameters can be measured and evaluated. Some of these parameters are as following: – RMS, MARSE, Peak, Signal to Noise Ratio, Integral, Kurtosis and … . Using acoustic emission technique for fault diagnosis of rotating machines is taken into account during last years. Using this technique beside the other condition monitoring techniques such as vibration analysis can be effective in timely or early detection of faults and REB’s life management as well. This technique can be very important in implementing proactive strategy in maintenance. BASAMAD has widely used this technique in machinery fault detection and has experienced many of case studies in this regard. These case histories are explained in acoustic emission course.