Abstract: There is provided a computer-implemented method of selectively communicating with mobile devices. At each of a plurality of mobile devices, a travel mode between a plurality of locations is determined in dependence on one or both of: one or more sensors associated with each respective mobile device, and wireless location signals received at the mobile device. At each mobile device, at least some of the plurality of locations are selected as significant locations associated with a user of the respective mobile device. Each significant location is a location at which the user resides for extended durations of time. Each mobile device communicates to a server computer an identifier associated with each mobile device, a generalised identifier associated with each significant location and an indication of the travel mode to the significant location.