Martial law is the temporary substitution of military authority for civilian government in an emergency. It involves the displacement of the civilian government by the military, where military commanders make laws, soldiers enforce laws, and ordinary citizens accused of defying martial law might face military tribunals instead of civilian courts. Martial law is usually declared in times of war, rebellion, natural disaster, civil unrest, or other emergencies. The circumstances in which it may be declared and other limiting factors vary by country.

During martial law, civil liberties such as the right to free movement, free speech, protection from unreasonable searches, and habeas corpus laws may be suspended. The justice system that typically handles issues of criminal and civil law is replaced with a military justice system, such as a military tribunal. Under total martial law, the normal American law enforcement and legal system is replaced by a stricter set of laws and punishments that is completely controlled by the military or executive branch of the government.

In the United States, martial law may be declared by the President or a State governor, but such a formal proclamation is not necessary. Although the U.S. Constitution makes no specific provision for the imposition of martial law, nearly every State has a constitutional provision authorizing the government to impose martial law. However, the power of martial law has limitations, for example, civilians may not be tried by military tribunals as long as civilian courts are functional. Nonetheless, within the bounds of court decisions, a military commanders authority under martial law is virtually unlimited.

To summarize, during martial law, military authority replaces civilian rule, and the military has unlimited authority to make and enforce laws. Civil liberties may be suspended, and the justice system is replaced with a military justice system. Martial law is usually declared in times of war, rebellion, natural disaster, civil unrest, or other emergencies. The power to declare martial law varies by country, and in the United States, it may be declared by the President or a State governor.