Cato's Letters were essays by British writers John Trenchard and Thomas Gordon, first published from 1720 to 1723 under the pseudonym of Cato (95–46 BCE), the implacable foe of Julius Caesar and a famously stubborn champion of republican principles (mos maiorum).
The Letters are considered a seminal work in the tradition of the Commonwealth men. The 144 essays were published originally in the London Journal, later in the British Journal, condemning corruption and lack of morality within the British political system and warning against tyranny.
The Letters were collected and printed as Essays on Liberty, Civil and Religious. A measure of their influence is attested by six editions printed by 1755. A generation later their arguments immensely influenced the ideals of the American Revolution; According to Peter Karsten's book Patriot-Heroes in England and America, Cato's Letters were the most common holdings on the bookcases of the founding fathers. 
Cato was used as a pseudonym by the Reverend Dr. William Smith, the most influential preacher in Philadelphia, in a series of essays arguing against American independence in the Pennsylvania Gazette published April 1776.
Cato was later appropriated as a pseudonym in a series of letters to the New York Journal in 1787 and 1788 opposing James Madison's views and urging against ratification of the United States Constitution. Many historians attribute these letters to George Clinton, though their authorship has not been definitively proven. These letters are unrelated to the Trenchard and Gordon letters.
These letters alsowspapers from Boston to Savannah, Georgia. Renowned historian Clinton Rossiter stated "no one can spend any time on the newspapers, library inventories, and pamphlets of colonial America without realizing that Cato's Letters rather than John Locke's Civil Government was the most popular, quotable, esteemed source for political ideas in the colonial period."
- John Trenchard and Thomas Gordon. Essays on Liberty, Civil and Religious, and Other Important Subjects. ed. and annotated by Ronald Hamowy. 2 vols. (Indianapolis, Liberty Fund, 1995). The standard modern edition.
- Karsten, Peter. 1978. Patriot-Heroes in England and America. The University of Wisconsin Press. Pages 34-5.
- Paine, Thomas. Letter. Pennsylvania Magazine 1776: n.p. Online Library of Liberty. Web. 21 Aug. 2013
- Cato #3
- Mitchell, Annie (July 2004). "A Liberal Republican "Cato"". American Journal of Political Science. 48. doi:10.1111/j.0092-5853.2004.00089.x.
- Rossiter, Clinton (1953). Seedtime of the Republic: the origin of the American tradition of political liberty. New York: Harcourt, Brace. p. 141.
- Cato Institute, "About Cato", undated, accessed January 2008.