http://www.epa.gov/ednnrmrl/publications/books/handbook/index.htm This handbook is intended to be a working document which assists scientists, engineers, consultants, regulators, citizen groups, and environmental managers in determining if stormwater runoff is causing adverse effects and beneficial-use impairments in local receiving waters. This includes adverse effects on aquatic life and human health and considers exposures to multiple stressors such as pathogens, chemicals, and habitat alteration. Given the complicated nature of the problem, where diffuse inputs contain multiple stressors which vary in intensity with time (and often in areas which are simultaneously impacted by point source discharges or other development activities, e.g., channelization), it is difficult to define and separate stormwater effects from these other factors. To accomplish this task requires an integrated watershed-based assessment approach which focuses on sampling before, during, and after storm events. This handbook provides a logical approach for an experimental design that can be tailored to address a wide range of environmental concerns, such as ecological and human health risk assessments, determining water quality or biological criteria exceedances, use impairment, source identification, trend analysis, determination of best management practice (BMP) effectiveness, stormwater quality monitoring for NPDES Phase I and II permits and applications, and total maximum daily load (TMDL) assessments. Despite the complexity of stormwater, successful and accurate assessments of its impact are possible by following the logical integrated approaches described in this handbook.