Rerum novarum, or Rights and Duties of Capital and Labor, is an encyclical issued by Pope Leo XIII on 15 May 1891. It is an open letter, passed to all Catholic patriarchs, primates, archbishops and bishops, that addressed the condition of the working classes.

Pope Leo XIII was head of the Catholic Church from 20 February 1878 until his death in July 1903. Living until the age of 93, he was the oldest pope, whose age can be validated, holding office and had the fourth-longest reign of any pope, behind those of St. Peter, Pius IX and John Paul II.