Pope Leo XIII reigned as Pope from February 20, 1878 to his death. He was the oldest pope (reigning until the age of 93), and had the third longest confirmed pontificate, behind that 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 labor unions, while affirming the rights of property and free enterprise, opposing both Atheistic Marxism and laissez-faire capitalism. He influenced Roman Catholic Mariology and promoted both the rosary and the scapular.

Leo XIII issued a record of eleven Papal encyclicals on the rosary earning him the title as the "Rosary Pope".