Wind energy has a major contribution to the catalog of renewable energy generation and also the most efficient energy solutions driver. As the requirement and utilization for wind energy persist to develop exponentially, reduction in cost of O&M and reliability growth are the prime concerns in the maintenance strategies of wind energy conversion system (WECS). Failure of WECS results in wind turbine (WT) shutdown or expenses maintenance that extensively cut down the yearly revenue. The WECS failures result in frequent planned O&M scheduled to guarantee the reliability of wind power (WP) generation. So, condition monitoring (CM) and fault detection and diagnosis (FDD) methodology has been universally introduced for the premature fault detection in order to reduce downtime period and increase generation. If at early stages detestation and rectification are not performed, faults may lead to disastrous state with large loss of revenue. Therefore, CM and FDD of WECS’s each equipment (i.e., rotor, gearbox, drive trains, generators, and power electronics) is the need of future research. This chapter introduces and provides an analysis of the current state of CM and FDD for each key element in WECS. This work also introduces the survey with numerous possible paybacks of CM for WECS.