The position that taxation is theft, and therefore immoral, is found in a number of political philosophies. Its popularization marks a significant departure from conservatism and classical liberalism, and has been considered radical by many as a result. The position is often held by anarcho-capitalists, objectivists, most minarchists, right-wing libertarians, and voluntaryists, as well as left-anarchists, libertarian socialists and some anarcho-communists.

Theft is the act of taking another person's property or services without that person's permission or consent with the intent to deprive the rightful owner of it. The word theft is also used as a synonym or informal shorthand term for some crimes against property, such as larceny, robbery, embezzlement, extortion, blackmail, or receiving stolen property. In some jurisdictions, theft is considered to be synonymous with larceny, while in others, theft is defined more narrowly. A person who engages in theft is known as a thief.