Rerum novarum (from its incipit, with the direct translation of the Latin meaning "of revolutionary change"), 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.

It discusses the relationships and mutual duties between labor and capital, as well as government and its citizens. Of primary concern is the need for some amelioration of "the misery and wretchedness pressing so unjustly on the majority of the working class". It supports the rights of labor to form unions, rejects both socialism and unrestricted capitalism, while affirming the right to private property.

Rerum Novarum is considered a foundational text of modern Catholic social teaching. Many of the positions in Rerum novarum are supplemented by later encyclicals, in particular Pius XI's Quadragesimo anno (1931), John XXIII's Mater et magistra (1961) and John Paul II's Centesimus annus (1991), each of which commemorates an anniversary of the publication of Rerum novarum.

Pope Leo XIII (Italian: Leone XIII; born Vincenzo Gioacchino Raffaele Luigi Pecci; 2 March 1810 – 20 July 1903) was the head of the Catholic Church from 20 February 1878 to his death in 1903. Living until the age of 93, he was the oldest pope (with the exception of Pope Benedict XVI as emeritus pope), and had the third-longest confirmed pontificate, behind those of Pius IX (his immediate predecessor) and John Paul II.

He is well known for his intellectualism and his attempts to define the position of the Catholic Church with regard to modern thinking. In his famous 1891 encyclical Rerum novarum, Pope Leo outlined the rights of workers to a fair wage, safe working conditions, and the formation of trade unions, while affirming the rights of property and free enterprise, opposing both socialism and laissez-faire capitalism. With that encyclical, he became popularly titled as the "Social Pope" and the "Pope of the Workers", also having created the foundations for modern thinking in the church's social doctrine, influencing the thoughts of his successors. He influenced Mariology of the Catholic Church and promoted both the rosary and the scapular. Upon his election, he immediately sought to revive Thomism, the theology of Thomas Aquinas, desiring to refer to it as the official theological and philosophical foundation for the Catholic Church. As a result, he sponsored the Editio Leonina in 1879.

Leo XIII is particularly remembered for his belief that pastoral activity in the socio-political field was also a vital mission of the church as a vehicle of social justice and maintaining the rights and dignities of the human person.

Leo XIII issued a record of eleven papal encyclicals on the rosary, earning him the title of the "Rosary Pope". In addition, he approved two new Marian scapulars and was the first pope to fully embrace the concept of Mary as Mediatrix. He was the first pope never to have held any control over the Papal States, which had been dissolved by 1870. Similarly, many of his policies were oriented towards mitigating the loss of the Papal States in an attempt to overcome the loss of temporal power, but nonetheless continuing the Roman Question.

At his death in 1903 he was briefly buried in the grottos of Saint Peter's Basilica before his remains were later transferred in 1924 to the Basilica of Saint John Lateran.