Congress occupies the field when it preempts state authority in an area of law through federal law so pervasive as to make reasonable the inference that Congress intended to supersede state law by leaving no room for it. Congress occupies the field when the federal government’s interest is so dominant that the federal system will be assumed to preclude enforcement of state laws on the same subject.
The Supreme Court of the United States elaborates on the notion in Pacific Gas & Electric Co. et al. v. State Energy Resources Conservation and Development Commission et al., which involved a question on whether the Federal Atomic Energy Act preempted California’s moratorium on certifications of new nuclear power plants.
[Last updated in July of 2020 by the Wex Definitions Team]