In English law certiorari (Latin, "to inform") is a public law relief (i.e. something for which you ask the court in order to deal with an action of the Government, council or other (quasi)-governmental organisation.) See judicial review and writ. An order of certiorari is given by a senior court to reverse the actions of a lower court or other (quasi)-governmental organisation which has made a decision. The use of certiorari in the UK is declining, due to the changes to the remedies available for judicial review.

Historically, certiorari was a prerogative writ used to direct a lower court or tribunal to certify for review the "record" in the case.

In the U.S., certiorari is the writ that an appellate court issues to a lower court in order to review its judgment for legal error, where no appeal is available as a matter of right. Since most cases cannot be appealed to the U.S. Supreme Court, for example, a party who wants that court to review a decision of a federal or state court files a "petition for a writ of certiorari" in the Supreme Court. If the court grants the petition, the case is scheduled for briefing and argument. That does not necessarily mean the Supreme Court has found anything wrong with the decision, merely that it wants to look at it for some reason. Four of the nine justices must vote to grant a writ of certiorari. This is called "the rule of four".. The great majority of cases brought to the Supreme Court are denied certiorari, because the Supreme Court is generally careful to choose only cases in which it has jurisdiction and which it considers sufficiently important to merit the use of its limited resources.

The legal effect of the Supreme Court's denial of a petition for a writ of certiorari is commonly misunderstood as meaning that the Supreme Court approves the decision of a lower court. However, such a denial "imports no expression of opinion upon the merits of the case, as the bar has been told many times." Missouri v. Jenkins, 515 U.S. 70 (1995).

Certiorari is sometimes informally referred to as cert, and cases warranting the Supreme Court's attention as certworthy. One situation where the Supreme Court sometimes grants certiorari is when the federal appeals courts in two (or more) federal judicial circuits have ruled different ways in similar situations, and the Supreme Court wants to resolve that "circuit split" about how the law is supposed to apply to that kind of situation.

Some U.S. state court systems use the same terminology, but in others, writ of review or leave to appeal is used in place of writ of certiorari as the name for discretionary review of a lower court's judgment. A handful of states lack intermediate appellate courts; their supreme courts operate under a mandatory review regime, in which the supreme court must take all appeals in order to preserve the loser's traditional right to one appeal.

In the administrative law context, the common-law writ of certiorari was historically used by lower courts in the U.S. for judicial review of decisions made by an administrative agency after an adversarial hearing. Some states have retained this use of the writ of certiorari, while others have replaced it with statutory procedures. In the federal courts, this use of certiorari has been abolished, and replaced by a civil action under the Administrative Procedure Act in a United States District Court, or in some circumstances, a petition for review in a United States Court of Appeals.