After discussing the nature of timeliness and its importance to the user of information systems, this document examines the factors which affect timeliness in three subsystems of the scientific communication system. The first subsystem moves a document from the existence of new knowledge to the point of entry into an information system; the second takes it from point of entry to completed registration in searchable files; and the third involves the process of searching and dissemination. Factors which affect the first subsystem include verification lag, manuscript writing, refereeing and editing. The second and third subsystems are examined in terms of three types of information systems: Retrospective Search Services (RSS), Selective Dissemination of Information (SDI), and Libraries. (WBC)