The U.S. Constitution established only one federal court—the U. S. Supreme Court. In lieu of creating other adjudicative bodies through the nation's founding document, Article I of the Constitution instead authorizes Congress to, in its discretion, "constitute Tribunals inferior to the [S]upreme Court." In the years following the ratification of the Constitution, Congress has regularly exercised its power to create a host of different federal tribunals that adjudicate a variety of legal disputes. For example, staffed by judges with lifetime tenures and salary protections, 13 federal circuit courts of appeals and over 90 federal district courts have been established by Congress under Article III of the Constitution. In addition to the judges who staff those courts, there are thousands of other judges, including administrative law judges, military judges, and federal magistrates who serve on non-Article III tribunals created by Congress. Notwithstanding the seemingly broad authority vested in Congress to establish federal courts, the Constitution does provide often sharp limits on when Congress can choose to create a federal tribunal to adjudicate a particular legal dispute. And the scope of these constitutional limits has been the focus of much debate, as evidenced by a long line of divided Supreme Court decisions on the subject. Indeed, as one legal scholar remarked, the law respecting federal courts and in particular the law distinguishing the powers of the various federal courts is "notoriously unfathomable." This report provides an overview of this often difficult and misunderstood area of law, beginning with a discussion of the various types of federal tribunals. The report continues by noting the rationales for why Congress established the breadth of different courts that exist today and concludes with a discussion of the various factors and relevant issues that limit Congress's discretion in establishing federal courts. Article III, Section 1 of the Constitution provides that the "judicial power" of the United States shall be "vested in one supreme Court, and in such inferior Courts as the Congress may from time to time ordain and establish.... " Article III or "constitutional" courts are not, however, the only body that Congress can assign the task of adjudication to under federal law. Instead, the Supreme Court has long recognized that "the Constitution [gives] Congress wide discretion to assign the task of adjudication in cases arising under federal law to legislative tribunals." The two central types of federal "courts"—courts established under Article III and those tribunals that are not—differ in many respects, including with regard to their personnel, purposes, and powers. In order to understand these differences, this section describes the two types of federal courts, beginning with Article III Courts. Courts established pursuant to Article III are mainly defined by the three central constitutional provisions to which they are subject. First, a constitutional court can exercise the "judicial power of the United States" to resolve "cases" and "controversies" of nine designated categories. The Supreme Court has interpreted the "case-or-controversy" requirement of Article III to impose certain rules of justiciability, such as the prohibition on advisory opinions, the requirements of standing and ripeness, and the limitation on the ability of federal courts to decide "political questions." Second, a judge who serves on a constitutional court holds his position during "good behavior." While the Constitution does not explain what "good behavior" entails or how a federal judge's term can expire, the Supreme Court has adopted the view that the Good Behavior Clause guarantees life tenure to Article III judges, "subject only to removal by impeachment." Third, an Article III judge's compensation cannot be "diminished during their Continuance in Office." The Supreme Court has interpreted the Compensation Clause to prohibit both direct and indirect methods of lowering of an Article III judge's pay, barring laws that either "order[] a lower salary" for a federal judge or laws that enact a discriminatory tax that uniquely affects federal judges. The three central provisions respecting Article III courts are fundamental to the basic purposes of such courts in the American constitutional scheme. The Framers of the Constitution, while proponents of democracy, were wary of any form of unchecked power, even when that power was lodged in a democratic majority. As a consequence, the Framers envisioned a written Constitution, which protected specific values, principles, and rights, as a limit of what could be changed through ordinary political processes. Because the political branches naturally cannot be expected to fairly adhere to the near-permanent constitutional limitations placed on each body, as these branches are most directly responsive to the often temporary whims of the people, the federal judiciary established under Article III was deliberately designed by the Framers of the Constitution to be a "counter-majoritarian branch" that interpreted the written Constitution and protected its principles. The Constitution did this by "insulating the federal judiciary" from potential pressures, from either the political branches or the public, which could potentially "skew the decision making process or compromise the integrity or legitimacy of federal court decisions." The key sources of the judiciary "insulation" from the political processes are the Good Behavior Clause and the Compensation Clause of Article III. The Good Behavior Clause, by creating a "permanent tenure of judicial offices," ensures an "independent spirit in judges," and the Compensation Clause, by creating a "fixed provision for [the judiciary's] support," prevents the political branches from having power over a judge's subsistence and, with that, "power over his will." However, just as the Framers worried about the concentration of unchecked power in either of the political branches, so too did the founding generation have concerns regarding the reach of the judiciary. Indeed, the power that "belongs" to the judiciary, as articulated by Alexander Hamilton in Federalist No. 78, to "ascertain [the Constitution's] meaning as well as the meaning of any particular act proceeding from the legislative body" and, when faced with a conflict, "be governed by [the Constitution] rather than [a legislative act]," is an immense power. The power of judicial review, at bottom, entails the power of unelected officials to "apply and construe the Constitution, in matters of the greatest moment, against the wishes of a legislative majority, which is, in turn powerless to affect the judicial decision." Notwithstanding the scope of this power, the Framers of the Constitution were untroubled by the potential reach of the judiciary because Article III judges were limited to ruling in certain circumstances and could only exercise that power "when other actors—public officials and private citizens—created justiciable cases and controversies for them." As a consequence, as famously described by Alexander Hamilton, the Framers envisioned the judiciary as being the "least dangerous" branch of the government. Given the host of different types of constitutional courts, a fundamental question is when must a court be considered one that has been established under Article III and subject to Article III's restrictions. The answer to that question has produced, in the words of Justice John Marshall Harlan II, "much confusion and controversy." Perhaps the best answer to the question of when a court can be deemed an Article III court comes from the 1962 Supreme Court case of Glidden v. Zdanok . Glidden involved a challenge to a judgment issued in part by a judge of the Court of Claims while that judge was sitting by designation on the Second Circuit Court of Appeals. In an earlier case, Williams v. United States , the Supreme Court had held that the Court of Claims was not an Article III court because the matters being brought before the court were not of the type that an Article III court generally adjudicates: primarily monetary claims against the government. The Williams Court held as such, notwithstanding that the law creating the Court of Claims did not limit the tenure of judges on the court or provide the authority of the salaries of the judges on that court. In the intervening years since Williams, Congress had declared the Court of Claims was "created under Article III." Notwithstanding that declaration, the petitioner in Glidden argued, relying on Williams , that his constitutional right to have an Article III court adjudicate the breach of contract claim was violated because a judge from a non-Article III court was designated to the case on appeal. The Supreme Court issued a splintered decision in Glidden ultimately holding that the Court of Claims was an Article III court making the designation of the Court of Claims judge to the Second Circuit panel constitutionally valid. Justice Harlan, writing for a plurality of three, argued that Williams should be overturned because the question of whether a court is an Article III court does not turn on the nature of the court's subject matter, but instead on whether the court's "establishing legislation complies with the limitations" of Article III. In concluding that the Court of Claims was an Article III court, Justice Harlan noted that the establishing legislation complied with the three central constitutional provisions pertaining to constitutional courts—namely the Good Behavior Clause, the Compensation Clause, and the case-or-controversy requirement. Specifically, Justice Harlan noted that (1) Court of Claims judges had been given life tenure to ensure their independence; (2) Congress had not provided that the salary of a Court of Claims judge be subject to diminution; and (3) Congress had provided the Court of Claims with the authority to rule on "cases and controversies" by, for example, respecting the finality of the rulings of the court and by providing the court jurisdiction over justiciable matters. In other words, for Justice Harlan, what mattered in determining the status of the Court of Claims as an Article III court was not the nature of the court's subject-matter jurisdiction, nor an after-the-fact declaration by Congress that the Court was considered an Article III Court, but the nature of the enabling legislation for the court.