The objective of this manual is to gather and to synthesize the results of past documentation and research on highway stormwater runoff into a single-volume user's manual on water quality impact assessment and mitigation. The manual will be useful to highway designers and environmental professionals by presenting the available and appropriate impact prediction and mitigation tools for use during highway project planning and development activities. The development of this document had three components: a literature search on existing research results and operational findings on highway stormwater runoff that identifies existing water quality impact assessment and mitigation techniques; interviews with highway practitioners concerning the use of research results including maintenance of mitigation measures; and synthesis of the accumulated information into this single-volume, practical guidance manual. This manual is intended to be a self-contained desk reference for highway practitioners. An extensive bibliography is included, and the mathematical nomenclature is unified among the sections.