Control of messages in a publish/subscribe system is described. A publishing system creates a message for publishing to multiple subscribers via a broker system. The message and associated metadata are provided. The metadata defines control of the message and relates to control of publishing of the message, by the broker system, and/or control of use of the message, by a subscribing system. The message is published with the metadata. A broker system receives the message and analyses the metadata with respect to a subscriber. Based on the analysis, restrictions are applied to delivery of the message to the subscriber. A subscribing system receives the message and analyses the metadata with respect to the subscriber. Based on this second analysis, restrictions are applied to use of the message by the subscriber.