No society can make a perpetual constitution， or even a perpetual law.
—Thomas Jefferson （3rd President of the United States）
美国的每一个州都有自己的宪法，但是人们通常所说的宪法都指联邦宪法。1777年，美国联邦议会通过了《联邦条例》（ Articles of Confederation and Perpetual Union）。这是美国最早的宪法性法律文件。1787年5月，美国各州代表在费城召开制宪会议，同年9月通过了《美利坚合众国宪法》。1789年，美国宪法正式生效，后又附加了27条宪法修正案。
美国宪法规定实行联邦制，肯定了以立法、行政、司法三权分立，相互制衡为原则的资产阶级民主共和政体。200多年来，美国宪法的内容除通过宪法修正案的方式加以改变外，更重要的是通过联邦最高法院行使司法审查权，对联邦宪法作出解释，以及通过政党、总统和国会的活动所形成的宪法惯例来改变宪法的内容，以适应社会不断变化的需要。因此，美国宪法是世界上“寿命最长”的成文宪法。作为世界上第一部成文宪法，美国宪法也为以后许多国家宪法的制定提供了成功的范本。
United States Constitution
The Constitution of the United States of America is the supreme law of the United States. It is the foundation and source of the legal authority underlying the existence of the United States of America and the federal government of the United States. It provides the framework for the organization of the United States government and for the relationship of the federal government to the states， to citizens， and to all people within the United States.
The Constitution defines the three main branches of government： a legislature， in the form of a bicameral Congress； an executive branch led by the President； and a judicial branch headed by the Supreme Court. The Constitution specifies the powers and duties of each branch. The Constitution reserves all unenumerated powers for the respective states and the people， thereby establishing the federal system of government.
The United States Constitution was adopted on September 17， 1787， by the Constitutional Convention in Philadelphia， Pennsylvania， and ratified by conventions in each U.S. state in the name of“The People”. The Constitution has been amended twenty-seven times； the first ten amendments are known as the Bill of Rights.
The United States Constitution is the shortest and oldest written constitution still in use by any nation in the world today. The Constitution has a central place in United States law and political culture. The handwritten original document penned by Jacob Shallus is on display at the National Archives and Records Administration in Washington， D.C.
The Constitution as supreme law
The United States Constitution， a relatively simple document， is the self designated“supreme law of the land”. This clause is taken to mean that when state constitutions， or laws passed by state legislatures or the national Congress， are found to conflict with the federal Constitution， they have no force. Decisions handed down by the Supreme Court over the course of two centuries have confirmed and strengthened this doctrine of constitutional supremacy.
Final authority is vested in the American people， who can change the fundamental law if they wish， by amending the Constitution， or in theory， at least—drafting a new one. The people’s authority is not exercised directly， however. The day to day business of government is delegated by the people to public officials，both elected and appointed.
The power of public officials is limited. Their public actions must conform to the Constitution and to the laws made in accord with the Constitution. Elected officials must stand for election at periodic intervals， when their record is subject to intensive public scrutiny. Appointed officials serve at the pleasure of the person or authority who appointed them， and may be removed when their performance is unsatisfactory. The exception to this is the lifetime appointment by the President of Justices of the Supreme Court and other federal judges.
Most commonly， theAmerican people express their will through the ballotbox. The Constitution， however， does make provision for the removal of a public official from office， in cases of extreme misconduct or malfeasance， by the process of impeachment. ArticleⅡ， Section 4 reads：
“The President， Vice President， and all civil officers of the United States， shall be removed from office on impeachment for， and conviction of， treason， bribery， or other high crimes and misdemeanors.”
In such cases， the House of Representatives must vote a bill of impeachment. The public official is then tried in the Senate， with the Chief Justice of the United States presiding at the trial.
Impeachment is considered a drastic measure in the United States. In the past 200 years， only 13 U.S. officials have been impeached： nine judges， an Associate Justice of the Supreme Court， a Secretary of War， a Senator， and a President， Andrew Johnson.（ In the case of another President， Richard Nixon， although the House Judiciary Committee recommended impeachment the President resigned before a House vote was taken. ） Out of the thirteen cases， only four judges have been convicted and removed from office. State officials are similarly subject to impeachment by the legislatures of their respective states.
In addition to setting forth general political ideas， the Constitution provides the blueprint for the governmental system. The three major articles describe the three branches of the national government—legislative， executive and judicial—each with specific duties and responsibilities. Subjects on which the legislative branch can make laws are set out in considerable detail， although over the years judicial decisions have expanded the scope of congressional activity. The powers and duties of the President， as head of the executive branch， are described. A system of federal courts is outlined， and its relationship to other branches of government is set forth.
The principles of government
Although the Constitution has changed in many aspects since it was first adopted， its basic principles remain the same now as in 1789：
The three main branches of government are separate and distinct from one another. The powers given to each are delicately balanced by the powers of the other two. Each branch serves as a check on potential excesses of the others.