This paper identifies the benefits that two Oregon cities obtained from computerized water distribution models, used to prepare for the Stage 2 Disinfectants/Disinfection Byproducts (D/DBP) Rule and to identify operation improvements. These benefits include lessons learned from two successful dynamic simulation (or so-called "extended-period") modeling projects, as applied to water distribution systems. CH2M HILL and two Oregon utilities, Eugene Water and Electric Board (EWEB) and the City of Albany, took advantage of GIS applications, field work, SCADA, and hydraulic modeling to plan system improvements. EWEB, with a service area population of approximately 160,000, and the City of Albany, with a service area population of 43,000, both recognized the importance of using an extended period simulation (EPS) model to conduct the Initial Distribution System Evaluation (IDSE), required by the Stage 2 D/DBP Rule. Both utilities saw the first step as building and using an EPS to locate areas of high water age in their distribution systems. This paper provides key points to help water utility managers understand how two utilities chose to upgrade their existing steady-state models to dynamic models, with an eye towards meeting the new IDSE regulations. Challenges, benefits, and costs of the field work and the procedures for calibrating the models are described. For the field work procedures, pump flows and tank water surface changes measured by SCADA provided hourly changes in system demand over a period of 24 to 72 hours. Examples provided in this paper include how the models represented the actual water distribution system fluctuations during low demand periods, when water becomes stagnant in tanks and pipelines. The models help identify ways to reduce the water age and improve water quality. Includes tables, figures.