All machines, no matter how well they are designed, calculated, or produced will fail at some point of their existence. If the condition of the machines in use is not monitored in any way and no maintenance is carried out, the resulting failures my pose a large economic and safety risk. Even the small and at first not important faults can end in catastrophic measures. This paper describes the main faults that occur in electrical machines and the continuous diagnostic monitoring methods that can be used to detect these faults. Also, in general, online condition monitoring, diagnostic systems, online condition monitoring of hydrogenerators, basic signal processing of diagnostic monitoring systems, measurement and monitoring quantities, diagnostic monitoring of cavitation and monitoring of aggregate utility are also explained. All sensors required for monitoring the hydrogenerator are described, as well as their purpose.