Condition-based maintenance is the maintenance of a system, sub-system or equipment at the right place and at the right time. There are various techniques adopted for CBM such as vibrations analysis, oil analysis, wear particle analysis and other tribological methods. In addition there are more signal-based, model based and knowledge based approaches to lessen the need for maintenance and increase availability and reliability. Vibrations analysis is purely a signal based method and the idea of this research work was to thoroughly review the state-of-the-art in this technique and the goal was to obtain break-through results which are beyond the current state-of-the-art.