The need for regional frameworks for making assessments of environmental resources has been recognized for many years. However, attempts to define effective frameworks have met with only limited success. In the absence of a suitable available scheme, the U.S. Environmental Protection Agency developed, tested and applied approaches for defining regional frameworks to meet both multi- and single-purpose needs. Ecoregions were mapped to facilitate the assessment of existing patterns and trends in the extent and quality of environmental resources (particularly aquatic) and relationships with natural and human-related factors. Definition of these regions was based on patterns of combinations of characteristics including land use, land surface form, potential natural vegetation, and soils. Other hierarchical levels of these ecoregions, including subregions and aggregations of ecoregions, are currently being developed for assessing agricultural, forest, wetland, and surface water ecosystems. The definition of these regions is based on patterns of specific parameters, such as surface water alkalinity or total phosphorus in lakes, and apparent spatial associations between these patterns, and landscape characteristics including surficial and bedrock geology.