Ludwig Pastor, later Ludwig von Pastor, Freiherr von Campersfelden, was a German historian and a diplomat for Austria. He became one of the most important Roman Catholic historians of his time and is most notable for his History of the Popes. He was raised to the nobility by the Emperor Franz Joseph I in 1908 and was nominated for the Nobel Prize in Literature six times.

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.