The United States Environmental Protection Agency (EPA), with the assistance of the US Department of Energy (DOE) and the National Oceanographic and Atmospheric Admn (NOAA) has been evaluating the feasibility of an effects-based (critical loads) approach to atmospheric pollutant regulation and abatement. The rationale used to develop three of the six steps in a flexible assessment framework is presented along with a discussion of a variety of implementation approaches and their ramifications. The rationale proposes that it is necessary to provide an explicit statement of the condition of the resource that is considered valuable because: (1) individual ecosystem components may be more or less sensitive to deposition; (2) it is necessary to select indicators of ecosystem condition that can be objectively measured and that reflect changes in the quality of the assessment end point; and (3) acceptable status must be defined. The rationale also stresses the importance of defining the assessment regions and subregions to improve the analysis and understanding of the indicator response to deposition. Subregional definition can be based on a variety of criteria, including informed judgment or quantitative procedures. (Copyright (c) 1993 Springer-Verlag New York Inc.)