Abstract: There is provided a monitoring system for monitoring a plurality of mobile devices registered at a server, the monitoring system comprising the server and the plurality of mobile devices registered at the server, the plurality of mobile devices including a first mobile device, wherein each mobile device is arranged to acquire location data and to send the acquired location data to the server via a telecommunications network, the server storing a time series of location data for each device, wherein the server is configured to generate an alert message identifying the first mobile device in response to a time series of location data from the first mobile device satisfying a condition.