The report describes the general structure of the Intrex Retrieval Systems and each of the component subroutines. The report is not an introduction to Intrex. In addition to a general description, the report covers the following topics: (1) system architecture, (2) software details, (3) command control logic, (4) list manipulation logic, (5) Intrex utilities, (6) CTSS utilities, (7) data base generation, (8) supporting software and (9) Intrex beyond 1971. The appendices are: (A) Summary of Common References, (B) Data Base Formats, (C) The Intrex Environment, (D) Message Text, (E) Subroutine Linkages (called by), (F) Subroutine Linkages (calls), (G) Glossary of Intrex Terms, (H) Data Base Generation Procedure and (I) Index to Subroutines in Chapter III. (MM)