Ex parte Lange was a United States Supreme Court case that dealt with the issue of copyright infringement. The case involved a dispute between two authors, John S.D. Eisenhower and George H. Smith, over the authorship of a book. Eisenhower claimed that Smith had copied his work without permission and had infringed upon his copyright. The Supreme Court held that Smith had indeed infringed upon Eisenhower's copyright and that Eisenhower was entitled to damages. The Court also held that copyright infringement was a tort and that the damages awarded should be based on the value of the work that was copied. The Court also held that the copyright holder was entitled to an injunction to prevent further infringement. The Court's decision in Ex parte Lange established the principle that copyright infringement is a tort and that the copyright holder is entitled to damages and an injunction to prevent further infringement. The Court's decision also established the principle that the damages awarded should be based on the value of the work that was copied. This case has been cited in numerous subsequent cases and has been used to establish the legal principles of copyright infringement.

In Ex Parte Lange, the Supreme Court was asked to decide whether a state court had jurisdiction over an alleged copyright infringement. The majority opinion held that it did not have such jurisdiction because the Copyright Act of 1870 only gave federal courts exclusive authority in this area. Justice Field dissented from this decision and argued that Congress could not constitutionally grant exclusive jurisdiction to one set of courts while denying it to another. He further argued that if Congress wanted to limit the power of state courts in matters related to copyrights, then they should do so explicitly rather than relying on implication or inference from other statutes. Finally, he noted that there were no constitutional provisions preventing states from exercising their own powers with respect to copyright cases and thus any attempt by Congress would be unconstitutional interference with those rights.