1. Method of synchronizing a user's data processing device with data maintained on a data processing system to which the user's data processing device has an intermittent connection, including: determining by the data processing system, a first context based on parameter values continuously updated in substantially real-time on the basis of sensor measurement data, receiving at the data processing system an identification of the user of the data processing device, obtaining by the data processing system data linked to the identification of the user and invariant data associated with the user's data processing device, determining by the data processing system, a second context, valid over a longer term than the first context, based partially on said data linked to the identification of the user and in part on said invariant data associated with the user's data processing device, using both the first and the second context to control a synchronization process during which data is communicated between the data processing device and the data processing system, and which results in at least one of the data processing device or the data processing system obtaining the target set of data, wherein the composition of the target set of data is based on both the first and the second context.