An inquisitorial system is a legal system in which the court, or a part of the court, is actively involved in investigating the facts of the case. This is distinct from an adversarial system, in which the role of the court is primarily that of an impartial referee between the plaintiff or prosecution and the defense.

Inquisitorial systems are used primarily in countries with civil legal systems, such as France and Italy, or legal systems based on Islamic law like Saudi Arabia, rather than in common law systems. It is the prevalent legal system in Continental Europe, Latin America, African countries not formerly under British rule, East Asia (except Hong Kong), Indochina, Thailand, and Indonesia. Most countries with an inquisitorial system also have some form of civil code as their main source of law. Countries using common law, including the United States, may use an inquisitorial system for summary hearings in the case of misdemeanors or infractions, such as minor traffic violations.

The distinction between an adversarial and inquisitorial system is theoretically unrelated to the distinction between a civil legal and common-law system. Some legal scholars consider inquisitorial misleading, and prefer the word nonadversarial. The function is often vested in the office of the public procurator, as in China, Japan, and Germany.

In an inquisitorial system, the trial judges (usually plural in serious criminal cases) act as inquisitors who actively participate in the fact-finding process by questioning defense lawyers, prosecutors, and witnesses. They may also order specific pieces of evidence to be examined if they find the presentations by the defense or prosecution inadequate. Prior to the case getting to trial, magistrate judges (juges d'instruction in France) participate in the investigation of a case, often assessing material by police and consulting with the prosecutor.

The inquisitorial system applies to questions of criminal procedure at trial, not substantive law; that is, it determines how criminal inquiries and trials are conducted, not the kind of crimes for which one can be prosecuted or the sentences that they carry. It is most readily used in some 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 an adversarial system, judges focus on the issues of law and procedure and act as a referee in the contest between the defense and the prosecution. Juries decide matters of fact, and sometimes matters of the law. Neither judge nor jury can initiate an inquiry, and judges rarely ask witnesses questions directly during trial. In some United States jurisdictions, it is common practice for jurors to submit questions to the court that they believe were not resolved in direct or cross-examination. After testimony and other evidence are presented and summarized in arguments, the jury will declare a verdict (literally true statement) and in some jurisdictions the reasoning behind the verdict; however, discussions among jurors cannot be made public except in extraordinary circumstances.

Appeals on the basis of factual issues, such as sufficiency of the sum total of evidence that was properly admitted, are subject to a standard of review that is in most jurisdictions deferential to the judgment of the fact-finder at trial, be that a judge or a jury. The failure of a prosecutor to disclose evidence to the defense, for example, or a violation of the defendant's constitutional rights (legal representation, right to remain silent, an open and public trial) can trigger a dismissal or re-trial. In some adversarial jurisdictions (e.g., the United States, and England and Wales), a prosecutor cannot appeal a "not guilty" verdict (absent corruption or gross malfeasance by the court).

In adversarial systems, the defendant may plead "guilty" or "no contest," in exchange for reduced sentences, a practice known as plea bargaining, or a plea deal, which is an extremely common practice in the United States. In theory, the defendant must allocute or "voice" his or her crimes in open court, and the judge must believe the defendant is telling the truth about his or her guilt. In an inquisitorial system, a confession of guilt would not be regarded as ground for a guilty verdict. The prosecutor is required to provide evidence supporting a guilty verdict. But this requirement is not unique to inquisitorial systems, as many or most adversarial systems impose a similar requirement under the name corpus delicti.