Totalitarianism is a political concept of a mode of government that prohibits opposition parties, restricts individual opposition to the state and its claims, and exercises an extremely high degree of control over public and private life. It is regarded as the most extreme and complete form of authoritarianism. Political power in totalitarian states has often been held by rule by one leader which employ all-encompassing propaganda campaigns broadcast by state-controlled mass media. Totalitarian regimes are often marked by political repression, personality cultism, control over the economy, restriction of speech, mass surveillance and widespread use of state terrorism. Historian Robert Conquest describes a "totalitarian" state as one recognizing no limits to its authority in any sphere of public or private life and which extends that authority to whatever length feasible.