Ex Parte The Milwaukee Railroad Company was a case heard by the United States Supreme Court in 1866. At issue was whether or not the state of Wisconsin had authority to tax certain bonds issued by the railroad company, which were secured by mortgages on its property. The court held that while states have broad powers to tax real and personal property within their borders, they do not have power to impose taxes on securities issued outside of their jurisdiction. Furthermore, it found that such taxation would be unconstitutional as it would interfere with interstate commerce and violate due process rights under the Fourteenth Amendment. As such, Wisconsin's attempt at taxing these bonds was deemed invalid and unenforceable.

In Ex Parte The Milwaukee Railroad Company, the Supreme Court was asked to decide whether a state court had jurisdiction over an interstate railroad company. Justice Field delivered the dissenting opinion in which he argued that Congress had exclusive authority over interstate commerce and thus could not be regulated by any individual state. He further noted that while states may have some power to regulate certain aspects of railroads within their borders, they cannot interfere with or impede operations across multiple states. In conclusion, Field stated that it would be unconstitutional for a single state's laws to apply beyond its own boundaries and therefore the lower court did not have jurisdiction in this case.