To assist water engineers to utilize an advanced water distribution system optimizer, a user-friendly interface, database support, and mapping utilities have been integrated into ArcView GIS Version 3.1 using Avenue and the Dialog Designer extension. This decision support system is currently being developed into an ArcView GIS extension called WADSOP--WAter Distribution System Optimization Program. WADSOP optimized pipe sizing and layout to improve cost-effectiveness and reliability over most existing water distribution models based on less effective pipe simulation algorithms. GIS provides functions for development and preparation of accurate spatial information for input into the network design optimization model, which includes network layout, connectivity, pipe characteristics and cost, pressure gradients, demand patterms, cost analysis, network routing and allocation, and effective color graphic display of results.