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The United States’ system of government is a federalist system with a national government and fifty sovereign states. Any power not delegated to the federal government in the U.S. Constitution, nor prohibited by it to the states, are reserved to the states. Nonetheless, the powers of the federal government are extensive. Both the national government and each state government are divided into executive, legislative, and judicial branches. Written constitutions, both federal and state, form a system of separated powers, checks, and balances among the branches.

The executive branch of the U.S. federal government includes the President, the Vice President, the Cabinet and all federal departments, and most governmental agencies. All executive power of the government is vested in the President, who serves a four-year term. All federal legislative powers are vested in the Congress of the United States, which consists of two chambers; a Senate (100 senators, two from each fifty states serving a six-year terms) and a House of Representatives (435 members, which are apportioned by population, with each state guaranteed at least one Representative, and serving two year terms).

The legislative process can briefly be described as follows. To become law, a bill must be passed by both the House and the Senate, and signed by the President. The President has the option of vetoing the legislation, but the Congress can override the veto with a two-thirds vote of both chambers. Every state has a legislature with two chambers, except Nebraska, which has only one. Unlike federal laws, state laws only apply within a state's borders. The federal Constitution, federal laws, and international treaties are supreme to state or local law. Therefore, state and local laws that contradict federal laws or treaties are pre-empted and can be declared unconstitutional by a federal court.

The U.S. judicial system is hierarchical both at the federal and state level. The highest court in the United States, and the only one required by the Constitution, is the United States Supreme Court. The Supreme Court's decision is the final decision in any case. It has limited original jurisdiction, hearing most of its cases on appeal. In most cases, the losing party must petition the Court through a writ of certiorari to hear the case. The Court votes first on the writ, requiring four yes votes to hear the case.

The Circuit Courts of Appeals are the level of courts immediately below the Supreme Court. The Circuit Courts hear appeals from the District Courts, the lowest level of federal courts. There is one Federal Circuit Court, one D.C. Circuit Court, 11 other Circuit Courts, and more than 90 District Courts. The Congress has also established several courts that address special types of cases. Cases from these courts are typically appealed directly to the Supreme Court.

All federal judges are appointed by the President and approved by the Senate. Federal judges hold their office for life, subject to impeachment by Congress. They have authority to interpret the Constitution, all federal statutes, treaties, and federal administrative rules, but their authority is limited to actual cases and controversies. The federal courts do not provide advisory opinions. The federal courts also have authority to naturalize persons as U.S. citizens and to settle certain legal disputes, such as: between citizens of different states; between two or more states; between individuals and the federal government; between states and the federal government; and, between states and foreign governments.

State systems are also made up of a Supreme Court (sometimes with a different name), usually an intermediate appellate court, and a series of lower courts or trial courts, sometimes including specialized courts. State judiciaries interpret state laws and apply them in specific disputes relating to state law; they determine whether a state crime has been committed; they evaluate the constitutionality of state laws under the state constitution; and they review the legality of state administrative rules under state statutes.

The Constitution of the United States provides the basis for the U.S. government, and guarantees the freedom and rights of all U.S. citizens. No laws may contradict any of the Constitution's principles and no governmental authority in the U.S. is exempt from complying with it. The federal courts have the sole authority to interpret the Constitution and to evaluate the federal constitutionality of federal or state laws. States also have constitutions, which are the supreme law within the state. State statutes must conform to the respective state's constitution. All state constitutions and legislation can be preempted by federal legislation or the federal Constitution.

The legal system of the United States is based on the common law. This also applies to every state, with the exception of Louisiana (which relies on the French civil code). Common law has no statutory basis and judges establish common law by applying previous decisions (precedents) to present cases. Although typically affected by statutory authority, broad areas of the law, most notably relating to property, contracts, and torts are traditionally part of the common law. These areas of the law are mostly within the jurisdiction of the states, and thus state courts are the primary source of common law. Federal common law is relatively narrow in scope, being limited primarily to clearly federal issues that have not been addressed by a statute.