Subversion, as the term is commonly used, the working from within a nation to overthrow the established government, particularly by weakening it through secret means. Subversion may be promoted by either revolutionaries or agents of an outside power. Often it is used by a nation to help conquer or gain control of another nation. An example of successful subversion was the takeover of Czechoslovakia in 1948 by that country's Communists, a minority party directed from the Soviet Union.

To turn the people against their government, a subversive group will typically try to advance a seemingly attractive ideology such as Communism, anarchism, or syndicalism. In another kind of subversion, sabotage and other terrorist acts are used to promote the breakdown of order as well as to discredit the government's ability to maintain it. Sedition, the advocacy of resistance to lawful authority, is also a kind of subversion. Espionage that aids an enemy country is a subversive activity, as is treason, the betrayal of one's country to the enemy. During wartime, a subversive group allied to the enemy is often called a “fifth column.”

In the United States, the Internal Security Act (McCarran Act) of 1950 is the principal antisubversion law. The Internal Security Division of the Department of Justice is in charge of enforcing antisubversion laws and prosecuting subversives. The Federal Bureau of Investigation is responsible for most investigative work involving subversion. In addition, state and local police departments investigate subversion.