Wind turbines, which are attracting attention as eco-friendly renewable energy, are huge mechanical structures requiring a lifetime of more than 20 years. A real-time condition monitoring system is essential for economical and effective operation of wind turbines. It also requires fault diagnosis and maintenance technologies based on condition monitoring. Most condition monitoring systems are being developed in accordance with the guidelines of international certification bodies, where the location and number of sensors, installation methods, operating procedures, and data processing techniques are presented. On the other hand, because wind turbines have different characteristics depending on the operating conditions, such as rotational speed, output power, and load, effective diagnosis techniques may vary depending on the target system. Therefore, the technical know-how of each operating organization needs to be considered in order to apply appropriate fault diagnosis techniques. This presentation introduces wind turbine condition monitoring and fault diagnosis and discusses future diagnostic techniques.