Pope Leo XIII (Italian: Leone XIII; born Gioacchino Vincenzo Raffaele Luigi Pecci; 2 March 1810 – 20 July 1903) was head of the Catholic Church from 1878 until his death in 1903. He had the fourth-longest reign of any pope, behind those of Peter the Apostle, Pius IX (his predecessor), and John Paul II.

Born in Carpineto Romano, near Rome, Leo XIII is well known for his intellectualism and his attempts to define the position of the Catholic Church with regard to modern thinking. In his 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 to property and free enterprise, opposing both atheistic socialism and laissez-faire capitalism. With that encyclical, he became popularly called the "Social Pope" and the "Pope of the Workers", also having created the foundations for modern thinking in the social doctrines of the Catholic Church, influencing his successors. He influenced the Mariology of the Catholic Church and promoted both the rosary and the scapular. Upon his election, he immediately sought to revive Thomism, the theological system of Augustine of Hippo and Thomas Aquinas, wishing to make it the official political, theological, and philosophical foundation of the Catholic Church. As a result, he sponsored the Editio Leonina in 1879.

Leo XIII is remembered for his belief that pastoral activity in political sociology is also a vital mission of the church as a vehicle of social justice and maintaining the rights and dignities of the human person. He issued a record eleven papal encyclicals on the rosary, earning him the title "Rosary Pope". He also approved two new Marian scapulars. He was the first pope never to have held any control over the Papal States, which had been dissolved by 1870, since Stephen II in the 8th century. Similarly, many of his policies were oriented toward mitigating the loss of the Papal States in an attempt to overcome the loss of temporal power, but nonetheless continuing the Roman Question. After his death in 1903, he was buried in the Vatican Grottoes. In 1924, his remains were transferred to the Archbasilica of Saint John Lateran.

Gioacchino Vincenzo Raffaele Luigi Pecci was born in Carpineto Romano, near Rome, the sixth of the seven children of Count Domenico Ludovico Pecci (2 June 1767 – 8 March 1833), Patrician of Siena, Colonel of the French Army under Napoleon, and his wife Anna Francesca Prosperi-Buzzi (1773 – 9 August 1824). His uncle Giuseppe Pecci was a protonotary apostolic and referendary of the Signature of Justice and died in 1806. His brothers included Giuseppe and Giovanni Battista or Giambattista Pecci (26 October 1802 – 28 March 1882), 1st Count Pecci (Comes Romanus by Papal brief in 1880), who married on 8 August 1851 Angela Salina (7 February 1830 – 9 October 1899) and had issue, and sister Anna Maria Pecci, wife of Michelangelo Pecci. Until 1818, he lived at home with his family "in which religion counted as the highest grace on earth, as through her, salvation can be earned for all eternity". Together with Giuseppe, he studied in the Jesuit College in Viterbo until 1824. He enjoyed Latin and was known to have written his own Latin poems at the age of eleven. Leo was a descendant of the Italian leader Cola di Rienzo on his mother's side.

His siblings were:

In 1824, he and Giuseppe were called to Rome, where their mother was dying. Count Pecci wanted his children near him after the loss of his wife and so they stayed with him in Rome and attended the Jesuit Collegium Romanum.

In 1828, the 18-year-old Vincenzo decided in favour of secular clergy, and Giuseppe entered the Jesuit order. Vincenzo studied at the Academia dei Nobili, mainly diplomacy and law. In 1834, he gave a student presentation, attended by several cardinals, on papal judgments. For his presentation, he received awards for academic excellence and gained the attention of Vatican officials. Cardinal Secretary of State Luigi Lambruschini introduced him to Vatican congregations. During a cholera epidemic in Rome, he assisted Cardinal Giuseppe Antonio Sala in his duties as overseer of all the city hospitals. In 1836, he received his doctorate in theology and doctorates of civil and Canon Law in Rome.

On 14 February 1837, Pope Gregory XVI appointed the 27-year-old Pecci as personal prelate even before he was ordained a priest on 31 December 1837 by the Cardinal Vicar Carlo Odescalchi. He celebrated his first Mass with his priest brother Giuseppe. Shortly thereafter, Gregory XVI appointed Pecci as Papal legate (provincial administrator) to Benevento, the smallest Papal province, with a population of about 20,000.