In some geographic locations, acidic deposition is known to be affecting surface water chemistry on both long-term and short-term time scales. Considerable research in the past decade has greatly improved our understanding of the biological effects of acidification, particularly the relationship between chronic chemical conditions and biological responses. In comparison, relatively little is known about the role that short-term acidification is having on the composition or functioning of aquatic biological communities. Despite this scientific uncertainty, it is generally presumed that short-term acidification ('episodes') can result in significant adverse effects on aquatic resources of interest, particularly fish communities. Recognizing episodes as a potentially important source of uncertainty in index-based estimates of acidic deposition effects on populations of lakes and streams, the EPA has initiated the Episodic Response Project (ERP). From an acidification perspective, the ERP is designed primarily to quantify this component of uncertainty in regional population estimates, and to determine the degree to which acidic episodes adversely affect fish populations.