A new software tool named UVCAT has been developed through a New York State Energy Research and development Authority (NYSERDA) project entitled "Optimization of UV Disinfection". The software provides simulations of ultraviolet (UV) system operation and performance as a function of time. Inputs to the software include water and flow data over time, UV validation data describing dose delivery and monitoring, lamp aging and fouling curves, UV system configuration data, data on dose pacing, and UV system O&M cost data. Outputs from the software include on/off status, power level, and lamp life for each bank in the UV system, power consumption for the UV system under actual and ideal dose pacing scenarios, daily and accumulated power costs, and offspec performance. All this information is provided as a function of time and can be used to assess if the UV system proposed for a given application is under or over sized and providing efficient dose pacing. The software also provides O&M and life cycle costs for the system under consideration. UVCAT provides a more accurate assessment of O&M and life cycle costs compared to standard approaches used by the industry because it better accounts for the true performance of the UV system. The software also provides novel approaches for sizing and operating UV systems that should optimize capital and O&M costs and provide better selection of UV systems for a given UV application. The software is currently being revised to provide a more complete estimate of offspec performance and its impact on public health protection. The software will be finalized Summer 2005 and provided to the water and wastewater industry for use through NYSERDA and AwwaRF. Includes tables, figures.