Martial law is the system of rules that takes effect (usually after a formal declaration) when a particular situation requires that a military authority take control of the normal administration of justice (and usually of the whole state).

Martial law is instituted most often when it becomes necessary to favour the activity of military authorities and organizations, usually for urgent unforeseen needs, and when the normal institutions of justice either cannot function or could be deemed too slow or to weak for the new situation, i.e. due to war or civil disorder, in occupied territory, or after a coup d'etat. The need to preserve the public order during an emergency is the essential goal of martial law.

Usually martial law reduces some of the personal rights ordinarily granted to the citizen, limits the length of the trial processes, and prescribes more severe penalties than ordinary law. In many countries martial law prescribes the death penalty for certain crimes, even if ordinary law doesn't contain that crime in its system.

In many countries martial law imposes particular rules, one of which is curfew. Often, under this system, the administration of justice is left to military tribunals, called courts-martial.

In United States law, martial law is limited by several court decisions handed down during the American Civil War and World War 2. In Ex Parte Milligan 71 US 2 1866, the United States Supreme Court held that martial law could not be instituted within the United States when its civilian courts are in operation.

In Canada in 1970, Prime Minister Pierre Trudeau invoked the War Measures Act in response to a terrorist-led kidnapping incident called the October Crisis which imposed a variant of martial law on the country.

On September 21, 1972, President Ferdinand Marcos issued Proclamation 1081 declaring martial law in the Philippines after a series of bombings and civil strife perpetrated by the Communist Party of the Philippines. By virtue of Proclamation No. 2045, martial law was lifted on January 17, 1981.

Martial law was introduced in Poland by the Communist regime on December 13, 1981 to prevent democratic movements (such as Solidarity) from gaining popularity and attendant political power in the country. Many democratic leaders, including Lech Walesa, were imprisoned. This state of affairs lasted until 1983. After the downfall of Communism in Europe, it was determined, that martial law had been declared in clear violation of Poland's constitution. It authorised the Council of State to declare martial law only between parliamentary sessions. The parliament had been in session when martial law was instituted.

Martial law existed in Taiwan, under the name of the "Period of Mobilization for Suppression of Communist Rebellion " from April 18, 1948 to May 1, 1991. It allowed for the Nationalist government, led by Chiang Kai-shek and his successors, to utilize dictatorial powers.

See also: military law, Military rule, state of emergency.