The paper summarizes the indicators proposed for a pilot study by the U. S. Environmental Protection Agency's (EPA) Environmental Monitoring and Assessment Program for lakes and streams (EMAP-LS). An additional lake and stream monitoring program is needed because current programs do not provide regional or national estimates of ecological condition or trends with known levels of confidence. Because national and regional monitoring must necessarily focus on spatial issues, the authors explain why and how they concentrate their sampling at particular index periods and in particular index locations. They provide 10 criteria for selecting response indicators and outline the 18 indicators selected and being developed for lakes and streams. Finally, the authors discuss how they propose to determine waterbody impairment through use of regional reference sites, regional experts, historical data, models, and empirical distributions of indicator values.