Pursuant to a charter from the State of Pennsylvania, the Monongahela Navigation Company (Monongahela) (plaintiff) built a lock and dam on the Monongahela River. Monongahela exacted tolls from the river’s commerce to fund improvements to the site. The United States (defendant) initiated proceedings to condemn the lock and dam in accordance with its power under the Takings Clause of the Fifth Amendment. In a trial, Monongahela’s compensation was limited to the actual value of the property. Monongahela appealed. The United States Supreme Court granted certiorari.