The Venerable Pope Pius XII (Latin: Pius PP. XII; Italian: Pio XII), born Eugenio Maria Giuseppe Giovanni Pacelli, reigned as Pope, head of the Roman Catholic Church and sovereign of Vatican City State, from 2 March 1939 until his death in 1958.

Before election to the papacy, Pacelli served as secretary of the Department of Extraordinary Ecclesiastical Affairs, papal nuncio and Cardinal Secretary of State, in which capacity he worked to conclude treaties with European and Latin American nations, most notably the Reichskonkordat with Nazi Germany. His leadership of the Catholic Church during World War II remains the subject of continued historical controversy.

After the war, Pius XII contributed to the rebuilding of Europe, and advocated peace and reconciliation, including lenient policies toward vanquished nations and the unification of Europe. The Church, flourishing in the West, experienced severe persecution and mass deportations of Catholic clergy in the East. In light of his protests, and his involvement in the Italian elections of 1948, he became known as a staunch opponent of communism.

Pius XII explicitly invoked ex cathedra papal infallibility with the dogma of the Assumption of Mary in his 1950 Apostolic constitution Munificentissimus Deus. His magisterium includes almost 1,000 addresses and radio broadcasts. His forty-one encyclicals include Mystici Corporis, the Church as the Body of Christ; Mediator Dei on liturgy reform; Humani Generis on the Church's position on theology and evolution. He eliminated the Italian majority in the College of Cardinals in 1946.