Perjury is lying or making verifiably false statements under oath in a court of law. Perjury is a crime because the witness has sworn to tell the truth, and for the credibility of the court, witness testimony must be relied on as being truthful. It is seen as a very serious crime as it seeks to usurp the authority of the courts, because it can lead to miscarriages of justice.

It also applies to witnesses who have affirmed they are telling the truth. (Affirmation is used by witness who are unable to swear to tell the truth. For example, in the United Kingdom a witness may swear on the Bible or other holy book. But if a witness has no religion, or does not wish to swear on a holy book, they may make an affirmation that they are telling the truth instead.) Perjury also applies when witnesses have made a statement under penalty of perjury, even if they have not been sworn or affirmed before an appropriate official. An example of this is the United States' income tax return, which, for public policy reasons, must be signed as true and correct under penalty of perjury.

Statements of interpretation of fact are not perjury because people often make inaccurate statements unwittingly and not deliberately. Individuals may have honest but mistaken beliefs about certain facts or their recollection may be inaccurate. Like all other crimes in the common law system, to be convicted of perjury you have to have had the intention (the mens rea) to commit the act, and to have actually committed the act (the actus reus).

In some countries such as France, suspects cannot be heard under oath and thus do not commit perjury, whatever they say during their trial.

Famous perjurers

Famous perjurers include:

• Jonathan Aitken, British politician, who was a member of John Major's cabinet, sentenced to 18 months imprisonment for perjury
• Jeffrey Archer, British novelist and politician, sentenced to 4 years imprisonment for perjury