Each federal judicial district has one U.S. attorney and one or more assistant U.S. attorneys. They are responsible for prosecuting defendants in criminal cases in the federal district courts and for defending the United States when it is sued in a federal trial court.

U.S. attorneys are appointed by the president and confirmed by the Senate. Nominees must reside in the district to which they are appointed and must be lawyers. They serve a formal term of four years but can be reappointed indefinitely or removed at the president's discretion. The assistant U.S. attorneys are formally appointed by the U.S. attorney general, although in practice they are chosen by the U.S. attorney for the district, who forwards the selection to the attorney general for ratification. Assistant U.S. attorneys may be fired by the attorney general.

In their role as prosecutors, U.S. attorneys have considerable discretion in deciding which criminal cases to prosecute. They also have the authority to determine which civil cases to try to settle out of court and which ones to take to trial. U.S. attorneys, therefore, are in a very good position to influence the federal district court's docket. Also, because they engage in more litigation in the district courts than anyone else, the U.S. attorneys and their staffs are vital participants in policy making in the federal trial courts.