When NEPA is applicable, there are three types of NEPA review: Categorical Exclusions; Environmental Assessments; and Environmental Impact Statements.
NEPA requires Federal departments and agencies to complete environmental reviews before decisions are made to entirely or partly finance, assist, conduct, regulate, or approve new or revised agency rules, regulations, plans, policies, programs, or specific projects
Statute or Regulations:
National Environmental Policy Act of 1969, 40 CFR 1500-1508 (Council on Environmental Quality implementing regulations), Section 309 of the Clean Air Act (mandating EPA review of EISs). See also, CEQ NEPA Guidance