Ex parte Yerger 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 case arose when a prisoner, Yerger, was held in federal custody in Mississippi and sought a writ of habeas corpus from the state court. The federal government argued that the state court did not have the authority to issue the writ, as the power to issue such writs was reserved to the federal government. The Supreme Court held that the state court did not have the authority to issue the writ of habeas corpus. The Court reasoned that the power to issue such writs was reserved to the federal government, and that the state court did not have the authority to interfere with the federal government's power. The Court also noted that the writ of habeas corpus was a fundamental right, and that the federal government had the exclusive power to protect this right. The Court's decision in Ex parte Yerger established that the federal government had exclusive authority to issue writs of habeas corpus, and that state courts did not have the authority to interfere with this power. This decision has been cited in numerous subsequent cases, and has been used to support the notion that the federal government has exclusive authority to protect fundamental rights.

In Ex Parte Yerger, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving an alleged violation of federal law. The majority opinion held that the state court did not have jurisdiction because it lacked authority under the Constitution and laws of Congress to hear such cases. However, Justice Field dissented from this ruling on two grounds: first, he argued that Congress had granted states concurrent jurisdiction in certain areas; and second, he contended that even if Congress had not done so explicitly, there was nothing preventing states from exercising their inherent power to protect citizens against violations of federal law. He concluded by stating that "the right which every citizen has to be protected in his person and property against any violation of Federal law is too important for us lightly or unadvisedly [to] deny."