Watershed and stormwater managers need modeling tools to evaluate alternative plans for water quality management and flow abatement techniques in urban and developing areas. A watershed-scale, decision-support framework that is based on cost optimization is needed to support government and local watershed planning agencies as they coordinate watershed-scale investments to achieve needed improvements in water quality. The U.S. Environmental Protection Agency (EPA) has been working since 2003 to develop such a decision-support system. The resulting modeling framework is called the System for Urban Stormwater Treatment and Analysis INtegration (SUSTAIN). The development of SUSTAIN represents an intensive effort by EPA to create a tool for evaluating, selecting, and placing BMPs in an urban watershed on the basis of user-defined cost and effectiveness criteria. SUSTAIN provides a public domain tool capable of evaluating the optimal location, type, and cost of stormwater BMPs needed to meet water quality goals. It is a tool designed to provide critically needed support to watershed practitioners at all levels in developing stormwater management evaluations and cost optimizations to meet their existing program needs. Due to the complexity of the integrated framework for watershed analysis and planning, users are expected to have a practical understanding of watershed and BMP modeling processes, and calibration and validation techniques.