An EIS is a comprehensive document that is prepared to describe and evaluate the effects from a proposed action on the environment. The National Environmental Policy Act of 1969 (NEPA) requires the Federal government to provide a detailed statement of impacts (known as an EIS) resulting from any major Federal action that has the potential to significantly affect the environment. A "Federal action" is an activity that is entirely or partly financed, assisted, conducted or approved by a Federal agency. The "environment" is defined as the natural and physical environment and the relationship of people with that environment. A change in consequence, resulting from the action(s) is considered an "impact". Impacts can be positive or negative or both. An EIS describes all impacts to the affected environment, including effects to the land, water, air, living organisms, as well as social, cultural, and economic aspects. NEPA requires an analysis of alternatives. An EIS also evaluates impacts resulting from any reasonable alternatives to the proposed action. It is a decision-making document in that it selects the preferred alternative after thoroughly evaluating these impacts.
Although NEPA applies to all actions carried out, assisted, or licensed by the Federal government, the act specifies when an EIS must be prepared and the Council for Environmental Quality (CEQ) regulations provide the recommended format and content. In accordance with the CEQ regulations, Section 1502.1, the EIS "shall provide full and fair discussion of significant environmental impacts and shall inform decision makers and the public of the reasonable alternatives which would avoid or minimize adverse impacts or enhance the quality of the human environment".
A tiered EIS is prepared when there is a need to have subsequent NEPA documents (either an EIS or an Environmental Assessment) after an initial EIS. For example, another NEPA document might be needed to address impacts that may result from a follow-on, site-specific action that is included in the overall program. The tiered EIS is prepared to eliminate repetitive analysis of the same issues. During a tiered EIS process, the subsequent document will concentrate on discussions and analysis specific to the follow-on action, but will only summarize and reference issues discussed in the original, broader document.