Rules of Decision Act
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The Rules of Decision Act requires that federal courts apply state law in their decisions, except when in conflict with federal law.
This act came from Section 34 of the Judiciary Act of 1789. It is now codified, in slightly different form, in .
It states that:
The laws of the several states, except where the Constitution or treaties of the United States or Acts of Congress otherwise require or provide, shall be regarded as rules of decision in civil actions in the courts of the United States, in cases where they apply.
Its interpretation, especially the meaning of "the laws of the several states," was central to the issue in Erie Railroad Co. v. Tompkins.
