Ex parte Woollen was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The petitioner, Woollen, was a prisoner in the custody of the United States Marshal for the District of Indiana. He had been convicted of a federal crime and was serving a sentence in the federal penitentiary. Woollen sought a writ of habeas corpus from the state court, claiming that his conviction was unconstitutional. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in federal custody. The Court reasoned that the writ of habeas corpus was a federal remedy and that the state court did not have the power to interfere with the federal government's authority to enforce its laws. The Court also noted that the writ of habeas corpus was a remedy that could only be used to challenge the legality of a person's confinement, and not to challenge the legality of a conviction. The Court's decision in Ex parte Woollen established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal custody. This decision has been cited in numerous subsequent cases and has become an important precedent in the area of federal-state relations.

In Ex Parte Woollen, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving an alleged breach of contract between two parties from different states. The majority opinion held that the state court did not have jurisdiction because it lacked diversity of citizenship among the parties involved in the dispute. Justice Field dissented and argued that under Article III Section 2 of the Constitution, Congress has authority to grant original jurisdiction to state courts in cases where there is no diversity of citizenship among those involved. He further argued that Congress had done so by passing legislation granting such power and thus, he concluded, this case should be heard by a state court as opposed to federal district or circuit courts.