The National Environmental Policy Act (NEPA) was passed in 1969. The purposes of the act are: "To declare a national policy which will encourage productive and enjoyable harmony between man and his environment; to promote efforts which will prevent or eliminate damage to the environment and biosphere and stimulate the health and welfare of man; to enrich the understanding of the ecological systems and natural resources important to the Nation; and to establish a Council on Environmental Quality."
In general, NEPA requires federal agencies to study the environmental impact of any proposed federal actions significantly affecting the human environment. Federal actions include such things as building projects, roads, oil and mining leases, parkland purchases and military activities. The required studies, called Environmental Impact Statements (EISs), must include a statement of (1) the environmental impact; (2) adverse environmental effects that cannot be avoided; (3) alternatives to the proposed action; (4) relationship between short-term use of the environment and long-term productivity; and (5) any irreversible and irretrievable commitments of resources.
Though NEPA might appear to have little bite - after all, it doesn't restrict the government from taking action like the Endangered Species Act, it only requires a study before action is taken - it has become the chief tool environmental activists use to halt federal projects. Federal courts, interpreting the mandates of NEPA broadly, have been very receptive to their arguments. Activists can successfully use NEPA to stop federal projects because the high cost of an EIS and the possibility of expensive, time-consuming litigation can often make a project too costly to pursue. While environmental activists consider NEPA an important tool to protect the environment, many non-environmental groups consider NEPA a wasteful layer of bureaucracy that empowers extremists to delay and even stop worthwhile federal projects.