This report contains the documentation and user's manual for the Extended Transport (EXTRAN) Block of the Storm Water Management Model (SWMM). EXTRAN is a dynamic flow routing model used to compute backwater profiles in open channel and/or closed conduit systems experiencing unsteady flow. It represents the drainage system as links and nodes, allowing parallel or looped pipe networks; weirs, orifices, and pumps; and system surcharges to be simulated. EXTRAN is used most efficiently if it is only applied to those parts of the drainage system which cannot be simulated accurately by simpler, less costly models. The EXTRAN manual is designed to give the user complete information on executing of the model both as a block of the SWMM package and as an independent model. Formulation of the input data is discussed in detail and demonstrated by seven example problems. Typical computer output is also discussed. Problem areas which the user may confront are described, as well as the theory on which the EXTRAN model rests. The manual concludes with a comprehensive discussion of the EXTRAN code.