In the United States, Constitutional law generally refers to the provisions of the United States Constitution, as interpreted by the United States Supreme Court. Early in its history, the Supreme Court in Marbury v. Madison assumed the power - called judicial review - to review federal laws passed by Congress and executed by the executive branch to decide whether those laws (or their application) conformed to the Constitution. The court later extended this power to review for constitutional conformity to all state laws.
When exercising judicial review, the Court will often seek to avoid conflict with the Congress or President by basing its decision on non-constitutional grounds. However, the court's assumed power to invalidate federal and state laws or actions has no counterpart in common or civil law.
The Supreme Court's interpretations of Constitutional law are binding on the legislative and executive branches of the federal government, on the lower courts in the federal system and on all state courts. This system of binding interpretations or precedents evolved from the common law system, where precedent binds lower courts. However, neither English common law courts or continental civil law courts generally have the power to declare legislation illegal or unconstitutional but only the power to interpret the law itself.
There are a number of related doctrines that, once raised by a party, the Supreme Court will examine before deciding on a constitutional question. Perhaps the most important of these is whether the court can avoid the constitutional question by basing its decision on a nonconstituional reason. For example, if a federal statue is on shaky constitutional footing but applied to the challenging party in a manner that does not implicate the basis for the constituional claim, the Supreme Court will not decide whether the statue might be unconstituional if it were applied differently. Or, when reviewing a decision of a state's highest court, the Court may avoid constitutional decision making if the state court's decision is based on an independent and adequate state ground of law. That is, even if the state court decided the question of state law incorrectly, the Supreme Court will not review that decision for its correctness.
There are also many related doctrines that federal courts in general and the Supreme Court in particular will consider before allowing a lawsuit to go forward. These implicate whether there is a case and controversy before the court and include proper standing of the parties, whether the case raises abstract, hypothetical or conjectural questions, whether the case is ripe for decision, or moot and thus past decision, or whether the question presented is a political question, unreviewable by the Court because the Constituion relegates it to another branch of government. These doctrines, because they apply to all federal cases whether of constitutional dimension or not, are discussed separately in a note on federal jurisdiction.
See also: United States Constitution