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An ex parte decision is one decided by a judge without requiring all of the parties to the controversy to be present. In U.S. and U.K. legal doctrine it means a legal proceeding brought by one person in the absence of and without representation or notification of other parties. It is also used more loosely to refer to improper unilateral contacts with a court, arbitrator or represented party without notice to the other party or counsel for that party.
In the United States, the availability of ex parte orders or decrees from both federal and state courts is sharply limited by the Fifth and Fourteenth Amendments, which provide that a person shall not be deprived of any interest in liberty or property without due process of law. In practice this has been interpreted to require adequate notice of the request for judicial relief and an opportunity to be heard concerning the merits of such relief. A court order issued on the basis of an ex parte proceeding, therefore, will necessarily be temporary and interim in nature, and the person(s) affected by the order must be given an opportunity to contest the appropriateness of the order before it can be made permanent. However, the Patriot Act, signed into law by Congress and President in 2001, 45 days after the September 11 attacks, allows in wartime for the arrest and ex parte trial of anyone deemed as aiding the enemy in any way (eg. terrorists). This actually includes (and specifies) foreigners, which was a major stumbling block previously in hunting down terrorists (they were not legalized Americans, therefore not as much could be done regarding them).
The phrase has also traditionally been used in the captions of petitions for the writ of habeas corpus, which were (and in some jurisdictions, still are) styled as "Ex parte Doe", where Doe was the name of the petitioner who was alleged to be wrongfully held. As the Supreme Court's description of nineteenth century practice in Ex Parte Milligan shows, however, such proceedings were not ex parte in any significant sense. The prisoner's ex parte application only sought an order requiring the person holding the prisoner to appear before the court to justify the prisoner's detention; no order requiring the freeing of a prisoner could be given until after the jailer was given the opportunity to contest the prisoner's claims at a hearing on the merits.