Effective implementation of predictive maintenance programs in power plants requires the online condition monitoring of electrical generators. This book offers a comprehensive guide on the measurement, detection, and interpretation of partial discharges in hydroelectric generators. It covers a range of essential topics such as the physics of partial discharge phenomenon, various types of defects and partial discharge patterns, sensors and acquisition procedures, signal processing techniques, automatic classification of discharge types, and correlation between partial discharge occurrence and ozone generation. Numerical modelling of partial discharges and calculation of the associated radiating electromagnetic fields are also discussed. To aid understanding, the book provides theoretical explanations, practical examples, and functional Python code on Google's Colaboratory platform. This book is a valuable resource for anyone seeking a deep understanding of partial discharges in hydroelectric generators.