This report is designed to provide an overview of the Environmental Protection Agency's (EPA) policies and technical guidance on the role of mixing zones in the NPDES permitting process, present state mixing zone regulations, policies, and guidance, introduce important concepts related to the hydrodynamics of effluent dilution in receiving waters and the design of outfall diffusers, review available mixing zone models, identify EPA sources for the models, discuss strategic issues for dischargers to consider when applying models, and describe the use of dye tracer studies as alternatives or supplements to mixing zone models.