Software license management systems are provided in which a license to use a software product is represented by a data token. The systems have a software controller for controlling use of the software product at a user device, and a license management server for communicating with the software controller via a data communications network. The software controller allows use of the software product at the user device substantially only during a use period associated with a current data token supplied to the software controller by the license management server. An exchange token can be supplied to another, similar software controller when necessary to transfer the license, e.g. in the event of a breakdown. The software controller is adapted to supply either the current data token, or the exchange token, to the license management server to be exchanged for a new data token.