Disclosed is a content delivery system of networked computer systems, the content delivery system for effecting transmission of media assets to a user device having an output interface available to a user, the media asset being transmitted as a media stream for outputting to the user while the media stream is still being received at the user device. The content delivery system comprises a first computer system operated by a first operator and a second computer system operated by a second operator. Each of the first and second computer systems is configured to execute a respective instance of contract software for effecting the transmission of the media asset to the user device as the media stream. The contract software embodies a contract between the first operator and the second operator defining permitted interactions of their computer systems.