In the majority opinion, Justice Anthony Kennedy said federal law allows the practice and that "this is not a case where the magistrate judge is asked to preside or make determinations after the trial has commenced." Justice Clarence Thomas dissented, saying, "Whatever their virtues, magistrate judges are no substitute" for U.S. District Court judges.

U.S. District judges are appointed by the president, confirmed by the Senate, have life tenure and their salaries cannot be reduced. Magistrates, appointed by U.S. District judges, have no such protection.

A Mexican citizen, Gonzalez does not speak fluent English and did not have an interpreter at the bench conference where his attorney consented to designating the magistrate for the task.

Ordinarily, U.S. District Court judges participate in selecting juries for criminal and civil trials, a process known as voir dire.

Lawyers regard voir dire as one of the most important parts of a case because it often uncovers biases, leading to excluding members of the group of citizens from which a jury is selected.

There are more than 500 federal magistrate judges, who serve eight-year terms and are recommended by a citizen's merit screening committee. The magistrate setup was begun by Congress in 1968, expanding on a system that is 175 years old.

Faced with increasing caseloads, the federal judiciary assigns an array of duties to magistrates. Those include initial proceedings in criminal cases, presiding over some minor criminal trials and trials of civil cases with the consent of both sides.