In a substitutional media system, one of a set of media items is selected at a programme receiver for output within a predefined programming slot, based on profile data available at the receiver. Sets of the media items are scheduled to slots according to one or more constraints pertaining to that slot and/or to media items scheduled to other slots. The receiver may select a media item for output in a slot by resolving constraints with media items scheduled to other slots, or selected for output in other slots.