A system, method and machine-readable instructions for monitoring a power electronics device. The system involves a semiconductor device, at least one sensor and a processor. The processor is configured to monitor a junction temperature of the semiconductor device by determining from the at least one sensor an on-state resistance of the semiconductor device and calculating the junction temperature of the semiconductor device according to a relationship between the on-state resistance of the semiconductor device and the junction temperature of the semiconductor device. The processor may apply an ageing coefficient to the on-state resistance.