Tiering is a process that tackles large problems and projects in levels. The broadest and most general issues are dealt with first, and each subsequent level deals with a more specific issue.

As described in the President's Council on Environmental Quality (CEQ) Regulations (40 CFR 1508.28) tiering refers to the coverage of general matters in broader Environmental Impact Statements (EIS's), with subsequent narrower environmental documents concentrating on more specific issues or proposals.

This approach is useful when considering the South Suburban Airport. More general matters such as need and site selection were dealt with first, in the Tier 1 EIS. The Tier 2 EIS will address construction of the inaugural airport, a more specific issue. Subsequent EIS's will deal with increasingly narrower subjects, and each requires approval in its own right.