Abstract: This disclosure relates to methods and apparatus for monitoring a remote system. In one arrangement, a plurality of measurement data units are obtained. Each measurement data unit represents a time series of measurements made by a sensor system at the remote system. A first trained machine learning model is used to identify a subset of the measurement data units that have a higher average probability of corresponding to an abnormal state of the remote system than the other measurement data units. Data representing the identified measurement data units is sent over a communications network to a central data processing system. An abnormal state of the remote system is detected by using a second trained machine learning model at the central data processing system to process the data representing the identified measurement data units.