A system controls access to a group of media items. A client device is operable by a user. A media server is coupled to at least one media data source, wherein the at least one media data source stores one or more media items for supply, by the media server, to the client device over a network. An authorization server is coupled to an authorization data source. The authorization data source stores data identifying a group of media items of fixed group size N that are accessible by the user, each media item in said group having a different associated availability parameter value indicating a number of time periods M within which access to the media item is available, wherein the authorization server is arranged to authorize the media server to supply a media item to the client device if said media item belongs to the group of media items.