Mister, usually written in its abbreviated form Mr. (US) or Mr (UK), is a commonly used English honorific for men under the rank of knighthood. The title 'Mr' derived from earlier forms of master, as the equivalent female titles Mrs, Miss, and Ms all derived from earlier forms of mistress. Master is sometimes still used as an honorific for boys and young men.

The modern plural form is Misters, although its usual formal abbreviation Messrs(.) derives from use of the French title messieurs in the 18th century. Messieurs is the plural of monsieur (originally mon sieur, "my lord"), formed by declining both of its constituent parts separately.