In the context of the Timbre Expert System, criteria consist in the set of information considered by the Artificial Neural Network methodology for ranking the documents downloadable from the web links provided as outputs of each search session. Specifically, criteria can be: user-related (referring to specific end-user’s characteristics stored in his profile), session-related (their values change from session to session and depend on end-user’s objective when using the Expert System), scores (related to the evaluations of each document provided by previous end-user’s) and statistics (related to number of “clicks” received by a web-link).