The Constitution says 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. The judges, both of the supreme and inferior courts, shall hold their offices during good behaviour, and shall, at stated times, receive for their services a compensation, which shall not be diminished during their continuance in office." (Source: Article 3, Section 1) The part about "good behaviour" is effectively a lifetime appointment, since it means that the judges can hold their offices for life unless convicted of a crime (which is fairly rare for American judges, possibly because of their being subject to removal in this way).
The de facto lifetime appointment here is among the most criticized parts of the Constitution, but was defended by the Founding Fathers as making them independent of having too much influence from the other branches (which could erode the separation of powers), or the popular prejudices of the people, who might otherwise have the power to retaliate against those who enforce laws against them - even when those laws are just and have actually been broken by the people who were judged against. In the words of Alexander Hamilton in the Federalist Papers, "The judiciary, on the contrary, has no influence over either the sword or the purse; no direction either of the strength or of the wealth of the society; and can take no active resolution whatever. It may truly be said to have neither force nor will, but merely judgment; and must ultimately depend upon the aid of the executive arm even for the efficacy of its judgments" (Source: Federalist No. 78) Hence, the radical mechanism of a de facto lifetime appointment, to protect against the improper influence of the other branches.
Supreme Court of the United States
The most important power possessed by the judiciary is the power to strike down laws that it deems "unconstitutional," or violating the spirit of the "supreme law of the land" that is the Constitution itself. This power is not explicitly granted to the courts by any passages in the Constitution itself; although it seems very clear that it is strongly implied in the Constitution, by certain passages limiting the power of the other branches. As an example, I will quote one of the clearest of them now, which limits the power of the Congress: "No bill of attainder or ex post facto law shall be passed." (Source: Article 1, Section 9, Paragraph 3) Before proceeding further, I should clarify that an "ex post facto" law is one that declares something you did on a prior occasion to be "illegal," even though it was perfectly legal at the time - thus making it illegal "after the fact" (or in the Latin phrase, "ex post facto"). A "bill of attainder" is a law passed by a legislature that declares a particular person (or persons) guilty of a crime and imposes a punishment, often without a trial - something that would usurp the proper power of the judicial branch, if it were allowed to be implemented (a truly frightening power, to the Founding Fathers and us).
Thus, Alexander Hamilton argued, what are the courts to do when faced with a law of this kind that is clearly unconstitutional? Are they to judge someone guilty of having violated this law anytime the executive branch prosecutes a case on such a basis, or are they to steadfastly and resolutely refuse to do so, on the grounds that these laws are clearly prohibited by the Constitution - or in other words, "unconstitutional"? The latter is clearly the case, as Alexander Hamilton once argued in the Federalist Papers: "The complete independence of the courts of justice is peculiarly essential in a limited Constitution. By a limited Constitution, I understand one which contains certain specified exceptions to the legislative authority; such, for instance, as that it shall pass no bills of attainder, no ex-post-facto laws, and the like. Limitations of this kind can be preserved in practice no other way than through the medium of courts of justice, whose duty it must be to declare all acts contrary to the manifest tenor of the Constitution void. Without this, all the reservations of particular rights or privileges would amount to nothing." (Source: Federalist No. 78)
Interior of the United States Supreme Court
But can't this power be abused, it might be argued, by declaring a law to be "unconstitutional" - when, in fact, it is not so at all? The answer is clearly "yes," but the only surefire way to prevent a power from being abused is to never grant anyone this power at all - something which clearly cannot be done, when there are certain explicit limits beyond which the legislative power cannot go (such as those mentioned in the Constitution); which cannot be enforced by any mechanism other than this power. There is only one "check" upon the judiciary, I am sad to say, which is to make sure that the power of appointing them to begin with is exercised wisely and well. Thus, a few words about that power of appointing them might well be in order here:
John Jay, the first chief justice of the Supreme Court (appointed to that position by George Washington)
The Constitution says that the president "shall nominate, and by and with the advice and consent of the Senate, shall appoint ambassadors, other public ministers and consuls, judges of the Supreme Court, and all other officers of the United States, whose appointments are not herein otherwise provided for, and which shall be established by law: but the Congress may by law vest the appointment of such inferior officers, as they think proper, in the President alone, in the courts of law, or in the heads of departments." (Source: Article 2, Section 2, Paragraph 2) This is why the controversies over the appointment of federal judges - and particularly, judges of the Supreme Court - are so heated and intense: the stakes are high, and the appointment is usually for life.
Thus, we must be careful about who we appoint to these judicial positions.
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The legislative branch: Two houses of Congress limited by a presidential veto
The executive branch: A single president subject to impeachment and removal
The Constitution: The "supreme law of the land" (which is difficult to amend)
Part of a series about
The Constitution itself, and the story behind it
Convention at Philadelphia: The writing of the Constitution (1787)
Preamble: The Constitution's mission statement, with some thoughts about separation of powers
The Congress: Its power to make laws, and the president's power to veto them (in some cases)
Frequency of elections: So how long do all of these people serve, anyway?
Representation: So who decides how many votes each state gets?
The presidency: Powers of the executive, and their being subject to impeachment
The courts: "Good behaviour," some important judicial powers, and how they're appointed
Miscellaneous: Amendment process, "supreme law of the land," and some closing remarks
Debates over the Constitution, then and since
Debates over ratification: Whether to adopt the Constitution in the first place
The "Federalist Papers": Frequently asked questions about them, and why they're important
Debates over checks & balances: Do they actually conflict with separation of powers?
The Bill of Rights: Important in the debates over ratification (adopted 1791)
The Constitution today: Some thoughts about civics education