This thesis is concerned with the investigation and development of methods to monitor the condition of electrical machines, with particular reference to induction machines fed from variable frequency, power electronic supplies. A variety of techniques are suggested whereby the faults that most commonly afflict electrical machines can be identified. Techniques involving the detection and analysis of line current, frame vibration, and leakage magnetic field are proposed and developed to take account of the rich spectrum of harmonics that can be injected through the use of power electronic inverters. To do this it was necessary to produce methodologies that could differentiate between spectral components originating in the inverter, and those produced as a result of the fault. As an extension, two further monitoring techniques were developed in an attempt to provide a fully comprehensive strategem for assessing the health of electrical drives. They are sequence component monitoring, and pulsed eddy current monitoring. The former is an adjunct to those techniques previously mentioned, and when used in combination with them leads to a greater security of diagnosis. The use of pulsed eddy currents is aimed at the on line detection of material failure in the rotating shafts of the machines. For each technique suggested, a comprehensive theoretical basis is provided, and its effectiveness verified by suitable experiment.