Constitutionality is the status of a law, a procedure, or an act's accordance with the laws or guidelines set forth in the applicable constitution. An act (or statute) which is enacted as law either by the council of a city or county, by the legislature of a state or province, or by a national legislature, may be declared unconstitutional. When the proper court determines that the act in question conflicts with the constitution, it finds that law unconstitutional and declares it void in whole or in part. This is called judicial review. The portion of the law that is declared void is considered to be struck down, or the entire statute is considered to be struck from the statute books.

Depending on the type of legal system, a statute may be declared unconstitutional by any court or only by special Constitutional courts which have the authority to rule on the validity of a statute.

In some countries, not having a formal written constitution, the legislature may create any law for any purpose and there is no provision for a law to be declared unconstitutional.

A constitutional violation is thus somewhat different from the breaking of a normal law, both in terms of seriousness and punishment. Declaring a law unconstitutional does not result in the punishment of those who passed it down.

In many jurisdictions the supreme court or constitutional court is the final legal arbiter that renders an opinion on whether a law or an action of a government official is constitutional.

Some examples of unconstitutional actions can be:

• A politician who abuses the powers of his constitutionally-established office.
• A legislature that tries to pass a law that would contradict the constitution, without first going through the proper constitutional amendment process.
• Any person acting on behalf of the government who tries to prevent an individual from exercising individual rights which the constitution protects (such as the right to vote or to practice religion).

Most constitutions define the powers of government. Thus, national constitutions typically apply only to government actions. This means that only governments can violate the nation's constitution, but there are exceptions.

The legal encyclopedia American Jurisprudence says the following in regard to constitutionality:

The general rule is that an unconstitutional statute, though having the form and the name of law, is in reality no law, but is wholly void and ineffective for any purpose since unconstitutionality dates from the time of its enactment and not merely from the date of the decision so branding it; an unconstitutional law, in legal contemplation, is as inoperative as if it had never been passed ... An unconstitutional law is void. (16 Am. Jur. 2d, Sec. 178)

Unconstitutional laws in the United States

Much debate often surrounds controversial laws enacted by state legislatures and the United States Congress regarding the laws' constitutionality. There are different forms of constitutions. The United States Constitution is a "rigid constitution". Rigid constitutions cannot be modified in their express terms, except through such processes the constitution itself ordains.