Generally, judicial review can be understood in two ways: first, as the power of a higher court to re-examine the factual or legal conclusions of a lower court or administrative body; and second, as the authority of the Supreme Court to assess the constitutionality of laws enacted by the legislature.

In the United Kingdom, judicial review pertains specifically to the judiciary's role in overseeing governmental activities based on public law principles that define its scope. This role empowers judges to regulate and evaluate the legality of actions taken by government bodies, tribunals, and lower courts (Cumper, p. 291). Judicial review, as a procedure within English Administrative Law, allows courts to oversee public authority actions. Individuals who believe an official decision—such as one made by a government minister, local council, or tribunal—violates their rights can seek judicial review in the Administrative Court, part of the High Court. However, unlike in the United States, English law does not permit judicial review of primary legislation (Parliament’s laws), except in cases where legislation contradicts EU law. While the courts can assess a law's compatibility with the Human Rights Act 1998, they cannot overturn or suspend a law found incompatible with the European Convention on Human Rights; they can only issue a declaration of incompatibility. The doctrine of parliamentary supremacy in the UK signifies that Parliament holds the ultimate legislative power, leaving the judiciary with no authority to review laws passed by Parliament.

While the interpretation of judicial review differs among jurisdictions, this discussion considers the term in a specific, narrower sense: the power of courts to oversee and control the legality of administrative agencies’ powers. Judicial review is fundamentally a mechanism through which courts ensure that public authorities operate within legal boundaries, upholding the rule of law (Wade & Forsyth, pp. 33-34). Its primary goal is to ensure that governmental authorities remain within their legal limits.

Judicial Review vs. Merits Review

Judicial review differs significantly from merits review in its purpose and scope. Merits review aims to evaluate whether an administrative decision is "correct and preferable," allowing the reviewing body to replace the initial decision if it finds it suboptimal. This review focuses on determining whether a decision was "right or wrong" based on a reassessment of all facts involved. Typically conducted by executive bodies, merits review allows the reviewing authority to "step into the shoes" of the original decision-maker. Administrative tribunals handle this type of review, typically without strict evidence rules, creating a less formal environment than courts. The power to conduct merits reviews may rest with courts (as an appeal), special tribunals, or general administrative tribunals, depending on the country’s practices.

In contrast, judicial review is a legal examination focused solely on the lawfulness of an administrative decision. When a court conducts judicial review, it assesses whether the decision aligns with the law, regardless of its "correctness" in other respects. If the court finds the decision lawful, it will not intervene, even if it might personally disagree with it. However, if the decision is deemed illegal or ultra vires (beyond lawful power), the court can set it aside and instruct the agency to reconsider it in line with the law. In judicial review, the court does not replace the original decision with its own but instead addresses legality alone. This approach was summarized in Attorney-General (NSW) v Quin (1990) 170 CLR at 35-36 per Brenan J., which emphasized that judicial review limits itself to declaring and enforcing the legal boundaries of administrative power, rather than remedying any perceived administrative errors or injustices.

A central tenet of judicial review is that "all power has its limits," and when administrative authorities exceed these limits, the judiciary has the authority to intervene. Judicial review does not necessarily prevent poor decisions but ensures that decisions are not made unjustly. This judicial restraint, limited to quashing decisions and remanding them back for reconsideration in line with legal guidelines, may at times be unsatisfying for both judges and those seeking relief. As Justice Peter McClellan observed, it is often the unfair process of making a decision, rather than the decision itself, that causes frustration and distress.

Unlike merits review, which originates from statutory law, judicial review is rooted in common law. Courts do not require statutory permission to conduct judicial review, as they exercise it as part of their inherent authority to uphold the law (Wade & Forsyth, p. 34). However, legislation can restrict this authority. In France, for example, ordinary courts lack the power to supervise government agencies, a role designated to administrative tribunals outside the regular judicial system. Some jurisdictions, however, grant statutory judicial review powers to ordinary courts, allowing them to supervise administrative decision-making to ensure compliance with the law.