An inquisitorial system is a legal system where the court or a part of the court is actively involved in determining the facts of the case, as opposed to an adversarial system where the role of the court is solely that of an impartial referee between parties. Inquisitorial systems are used in most countries in Western Europe and Latin America.

The inquisitorial system applies to questions of criminal procedure as opposed to questions of substantive law; that is, it determines how criminal enquiries and trials are conducted, not the kind of crimes for which one can be prosecuted, nor the sentences that they carry. It is most readily used in many, but not all civil legal systems. However, some jurists do not recognize this dichotomy and see procedure and substantive legal relationships as being interconnected and part of a theory of justice as applied differently in various legal cultures.

In some jurisdictions the trial judge may participate in the fact finding inquiry by questioning witnesses even in adversarial proceedings. The rules of admissibility of evidence may also allow the judge to act more like an enquirer than an arbiter of justice.

International tribunals intended to try crimes against humanity, such as the Nuremberg Trials and the International Criminal Court, have used the inquisitiorial system rather than the adversarial system.

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