With the enactment of the Clean Air Act amendments in 1970, the federal government, essentially an interested bystander as recently as 1960, became the dominant presence in air pollution control. The current statute provides for federal research, financial support of state control programs, and interstate compacts (secs. 101-106). It retains, in vestigial form, a cumbersome conference procedure (sec. 115) copied from the earlier water-pollution statute and never much used in air pollution. irect federal regulatory authority was substantially increased by the 1970 amendments: the federal Environmental Protection Agency (EPA) may now adopt emission standards not only for new vehicles (sec. 202) but also for all aircraft (sec. 231), for new stationary sources of any type that “may contribute significantly to air pollution” (sec. 111), and for any source of a “hazardous” air pollutant, that is, one that “may cause, or contribute to, an increase in mortality or … in serious irreversible, or incapacitating reversible, illness” (sec. 1 12); it may also regulate the contents of motor-vehicle fuels (sec. 211). An emergency provision authorizes the federal agency, absent adequate state or local action, to sue to enjoin any emissions contributing to “an imminent and substantial endangerment to the health of persons” (sec. 303). Yet everyday control of most existing stationary sources remains subject to an awkward joint federal-state system of air-quality standards. That system is the subject of the present article.