Sampling alone often provides an incomplete picture of water quality dynamics within a system and is of limited help when contemplating changes in system design and operation. EPANET is a third-generation public domain software developed by the US EPA's Risk Reduction Engineering Laboratory for modeling water quality within distribution systems. The program performs extended period simulation of hydraulic and water quality conditions within pressurized pipe networks. It tracks the flow of water in each pipe, the pressure at each pipe junction, the height of water in each tank, and the concentration of a dissolved substance at each junction during a multi-time period simulation. In addition to concentration, water age and source tracing can be simulated. The package actually consists of two programs. One performs the hydraulic and water quality simulations using data files for input and output reports. The second program provides a graphical user interface for interactively running the simulator and viewing its results via maps, data tables, and time series graphs. Major features of the program and how it can be used are listed.