Ex parte Zellner 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 a federal prison. The case arose when a prisoner, Zellner, was held in a federal prison in California and sought a writ of habeas corpus from a state court. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. 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 writ of habeas corpus was a federal prerogative, and that the state court did not have the authority to interfere with the federal government's power to imprison individuals. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to imprison individuals. The Court's decision in Ex parte Zellner established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal prisons. This decision has been cited in numerous subsequent cases, and has been used to support the notion that the federal government has the exclusive power to imprison individuals.

In Ex Parte Zellner, the Supreme Court was asked to decide whether a writ of habeas corpus should be issued in order for an individual to be released from custody. The majority opinion held that the writ should not issue because it would interfere with state proceedings and undermine public confidence in the judicial system. Justice Field dissented, arguing that there were no valid grounds for denying the writ since it did not involve any interference with state proceedings or prejudice public confidence. He further argued that if Congress had intended to limit habeas corpus relief, they would have done so explicitly rather than leaving such decisions up to judges who may lack sufficient information about particular cases. Field concluded by stating his belief that when a person is unlawfully detained, they are entitled to their freedom regardless of what other considerations might exist at play.