Judicial review is the doctrine under which legislative and executive actions are subject to review (and possible invalidation) by the judiciary. Specific courts with judicial review power must annul the acts of the state when it finds them incompatible with a higher authority (such as the terms of a written constitution). Judicial review is an example of the separation of powers in a modern governmental system (where the judiciary is one of three branches of government). This principle is interpreted differently in different jurisdictions, which also have differing views on the different hierarchy of governmental norms. As a result, the procedure and scope of judicial review differs from country to country and state to state.

Judicial review is one of the main characteristics of government in the Republic of the United States and other democracies. It can be understood in the context of two distinct—but parallel—legal systems (civil law and common law), and also by two distinct theories on democracy and how a government should be set up (the ideas of legislative supremacy and separation of powers). First, two distinct legal systems, civil Law and common law, have different views about judicial review. Common-law judges are seen as sources of law, capable of creating new legal rules, and also capable of rejecting legal rules that are no longer valid. In the civil-law tradition judges are seen as those who apply the law, with no power to create (or destroy) legal rules.