Treason is a criminal offence the attempt, by overt acts, to overthrow the government to which the offender owes allegiance, or to betray the state to a foreign power.

Two of treason existed in early English law: high treason, which was directed against the Crown, and petty treason, which of a crime against a subject, such as a wife killing her husband, or a servant murdering his master.

In early English statutes the more serious offences were compassing or imagining the death of the sovereign, adhering to the sovereign's enemies and giving them and comfort, and levying war against the sovereign. Statutes were changed from time to time between the reign of Edward III and that of Elizabeth I. After the the Stuart judges used " treason" to discourage resistance to the Crown. They extended the offences to include words as well as deeds. In 1663, a writer was convicted of treason for writing an article suggesting that the king was accountable to the people.

Treason is a rare offence in modern law. In England it is usually only in time of war, and the penalty is still death. In 1917, Roger Casement was executed for treason for attempting to gain German support for Irish independence during World War I; in 1946, William Joyce, who had broadcast from Germany throughout World War II and was known as "Lord Haw-Haw", was executed for treason. Joyce's conviction was as many believed that he was not a British citizen, having been born in the United States, having no passport, and no intention to be a British citizen.