Condition monitoring of engineering plants has increased in importance as engineering processes have become increasingly automated. However, electrical machinery usually receives attention only at infrequent intervals when the plant or the electricity generator is shut down. The economics of industry have been changing, placing ever more emphasis on the importance of reliable operation of the plants. Electronics and software in instrumentation, computers, and digital signal processors have improved our ability to analyse machinery online. Condition monitoring is now being applied to a range of systems from fault-tolerant drives of a few hundred watts to machinery of a few hundred MW in major plants.