In one embodiment there is disclosed a content delivery system comprising: a communications network; a media component connected to the communications network, the media component configured to deliver media content for playout at a user device; an auxiliary content component connected to the communications network for providing auxiliary content for playout at a user device; an engagement monitoring component configured to monitor user engagement with auxiliary content played out at the user device; a storage component which holds engagement monitoring data in association with user identifiers; a content control module configured to receive, from a user device, a request for auxiliary content to be inserted in a playout session for playout at the user device, and selectively control delivery of the auxiliary content based on the engagement monitoring data associated with a user identifier for the user device.