A content delivery system comprising: a communications network; a media component connected to the communications network, the media component configured to deliver content for consumption at user devices connected to the communications network; an engagement monitoring component configured to monitor user engagement with auxiliary content played out at the user devices; a storage component which holds engagement monitoring data in association with user identifiers of users of the content delivery system; a content control module configured to receive from a user device a request for content data, the content control module being operable to selectively deliver enhanced content data based on the engagement monitoring data held in association with a user identifier associated with a user of the user device.