WinTR-55 is a single-event rainfall-runoff small watershed hydrologic model. The model generates hydrographs from both urban and agricultural areas and at selected points along the stream system. Hydrographs are routed downstream through channels and/or reservoirs. Multiple sub-areas can be modeled within the watershed. Historical Background Technical Release 55 (TR-55) Urban Hydrology for Small Watersheds was first issued in January 1975 as a simplified procedure to calculate the storm runoff volume, peak rate of discharge, hydrographs and storage volumes required for storm water management structures (SCS, 1975). The first issue involved manual methods and assumed the NRCS Type II rainfall distribution for all calculations. In June 1986 major revisions were made in TR-55 by adding three rainfall distributions (Type I, IA and III) and programming the computations. Time of concentration was estimated by splitting the hydraulic flow path into separate flow phases (SCS, 1986). The computer program became a standard tool to analyze peak flow changes caused by urbanization in many locations. Its wide acceptance by public and private users has also indicated where improvements could be made in the procedures and the computer program. A WinTR-55 work group was formed in the spring of 1998 to modernize and revise the Technical Release and the computer software. The current changes include: upgrade the source code to Visual Basic, change the philosophy of data input, develop a Windows interface and output post- processor, enhance the hydrograph-generation capability of the software and flood route hydrographs through stream reaches and reservoirs. The availability and technical capabilities of the personal computer have significantly changed the philosophy of problem-solving for the engineer. Computer availability eliminated the need for WinTR-55 Guide methods, thus the guide portions (graphs and tables) of the user document have been eliminated. This user guide covers the procedures used in and the operation of the WinTR-55 computer program. Part 630 of the Natural Resources Conservation Service (NRCS) National Engineering Handbook provides detailed information on NRCS hydrology and is the technical reference for this document. Appendix C of the WinTR-55 User Guide (part of the install package) contains a list of the Part 630 chapters and their subjects. Users who are not familiar with NRCS hydrologic procedures should refer to the appropriate chapters for background information and the details of procedural techniques.