The State of California passed the Warren-Alquist Act to help regulate the storage and disposal of nuclear waste. Section 25524.1(b) of the Act provided that before a nuclear power plant could be built, the State Energy Resources Conservation and Development Commission (State Commission) (defendant) had to determine on a case-by-case basis that there would be "adequate capacity" for interim storage of the plant's spent fuel at the time the plant required that storage. Section 25524.2 of the Act imposed a moratorium on the certification of new nuclear plants until the State Commission found that there was a demonstrated technology or means, approved by the United States through its authorized agency, for permanently disposing of high-level nuclear waste. Pacific Gas & Electric Co. (PG & E) (plaintiff), a nuclear power producer, filed suit in federal district court seeking to enjoin enforcement of the California regulations. PG & E alleged that the regulations were preempted under the Supremacy Clause of the Constitution by the Atomic Energy Act of 1954 (AEA). The district court held that both § 25524.1(b) and § 25524.2 were preempted by the AEA. The appellate court reversed and held that § 25524.1(b) was not ripe for review, and that § 25524.2 was ripe for review but was not preempted by the AEA. The United States Supreme Court granted certiorari.