The drive-train and power electronics are critical for the operation of industrial wind turbines. Faults developing in these components can result in long downtime and expensive repair costs, particularly when offshore wind farms are concerned. Effective condition monitoring (CM) of these components can result in significant savings for wind farm operators and contribute to a substantial improvement of the operational reliability of wind turbines. This paper considers a novel modular CM system capable of diagnosing faults in the gearbox. The data analysis methodology and the key results arising from measurements on actual industrial wind turbines are also presented.