This manual presents six monitoring study designs that can be used by Municipal Separate Storm Sewer System (MS4) communities to assess their local stormwater programs. Limited information is currently available in most MS4 communities to determine how well their stormwater programs are functioning by quantifying their stormwater pollutant reductions to protect receiving water quality. The central purpose of this manual is to provide guidance to MS4 communities on developing monitoring studies whose results can help improve their local stormwater programs by getting more pollutant reduction out of the total community stormwater investment. Monitoring to evaluate the effectiveness of a stormwater program is becoming increasingly necessary. Communities are finding themselves in need of information to meet mandatory permit requirements (e.g. defined pollutant reduction goals) or to justify budgets that support stormwater programs. Monitoring is a requirement for Phase I MS4 communities and can be used to determine progress towards implementation for many of the other MS4 requirements listed in Table 1 for both Phase I and II communities. For example, Phase II communities are required to develop measureable goals to track progress towards implementing each of the six minimum management measures. Most communities' measureable goals are outputbased (e.g. number of stormwater treatment practices installed, number of educational brochures distributed), which is useful from a program accounting standpoint but does not allow changes in water quality as a result of these activities to be quantified.