This manual takes a "e;past, present, and future"e; look at the stormwater quality modeling industry. Stormwater quality models predict runoff volumes and loads from land surfaces and route them through receiving waterbodies. Some stormwater manuals that are referenced today are over 20 years old. Although the basic theory presented in these manuals remains applicable to today's stormwater industry, many of the water quality models described in older manuals are no longer used or have undergone significant redevelopment. This special publication presents state-of-the-art stormwater modeling in the United States. Advances in technology and the state of practice, along with more stringent permitting requirements, call for the improved use of geospatial and remotely sense acquired data and better integration of quality and quantity models to seamlessly exchange data and outputs between different decision support tools. This manual is a practical tool that presents a broad compilation of existing models, their capabilities, and a roadmap for users to assist in the selection of such models for diverse applications. Further background is given in this publication towards the history, evolution, and future of stormwater modeling practices. The audience for this manual are industry users with an underlying knowledge and understanding of the theory behind water quantity and quality modeling.