The mediation between bibliographic universe and user is a problem of communication. Catalographic indexes are an intermediate passage which counts different stages: description - relation between documents and their representation; order - relation between cataloguer and indexes; search - user and document representation interaction. The catalog is a tool of mediation and communication between readers and collecions, made by librarians. The catalog is a language that relates library with its users. The theory of cataloguing is analyzed through the figures of Panizzi, Cutter, Ranganathan, Lubetzky, Domanovszky, Malinconico, Maltese, Serrai, Dini, and Svenonius.