EPANET represents a third generation of water quality modeling software developed by the U.S. EPA's Drinking Water Research Division, offering significant advances in the state of the art for network water quality analysis. EPANET performs extended period simulation of hydraulic and water quality behavior within water distribution systems. In addition to substance concentration, water age and source tracing can also be simulated. EPANET includes a full featured hydraulic simulation model that can handle various types of pumps, valves, and their control rules. The water quality module is equipped to handle constituent reactions within the bulk pipe flow and at the pipe wall. It also features an efficient computational scheme that automatically determines optimal time steps and pipe segmentation for accurate tracking of material transport over time. EPANET is currently being used in the US to study such issues as loss of chlorine residual, source blending and trihalomethane (THM) formation, how altered tank operation affects water age, and total dissolved solids (TDS) control for an irrigation network.