The 1939 papal conclave was held, following the death of Pope Pius XI on 10 February 1939. All 62 cardinals of the Catholic Church met on 1 March. The next day, on the third ballot, they elected Cardinal Eugenio Pacelli, who was Camerlengo and Secretary of State, as pope. He accepted and took the name Pius XII. It was his 63rd birthday.

The conclave of 1939 was the shortest of the 20th century.Pacelli was the first member of the Roman Curia to become pope since Gregory XVI (1831) and the first Roman since Innocent XIII (1721).

A cardinal (Latin: Sanctae Romanae Ecclesiae cardinalis, literally "cardinal of the Holy Roman Church") is a senior member of the clergy of the Catholic Church, immediately behind the pope in the order of precedence. Collectively, they constitute the College of Cardinals, and are appointed for life.

Their most solemn responsibility is to elect a new pope in a conclave, almost always from among themselves (with a few historical exceptions), when the Holy See is vacant. During the period between a pope's death or resignation and the election of his successor, the day-to-day governance of the Holy See is in the hands of the College of Cardinals. The right to participate in a conclave is limited to cardinals who have not reached the age of 80 years by the day the vacancy occurs. In addition, cardinals collectively participate in papal consistories (which generally take place annually), in which matters of importance to the Church are considered and new cardinals may be created. Cardinals of working age are also appointed to roles overseeing dicasteries of the Roman Curia, the central administration of the Catholic Church.

Cardinals are drawn from a variety of backgrounds, being appointed as cardinals in addition to their existing roles within the Church. Most cardinals are current or retired bishops or archbishops leading dioceses around the world – often the most prominent diocese in their country. Others are titular bishops who are current or former officials within the Roman Curia (generally the leaders of dicasteries and other bodies linked with the Curia). A very small number are priests recognised by the pope for their service to the Church; as canon law requires them to be generally consecrated as bishops before they are made cardinals, but some are granted a papal dispensation. There are no strict criteria for elevation to the College of Cardinals. Since 1917, a potential cardinal must already be at least a priest, but laymen have been cardinals in the past. The selection is entirely up to the pope and tradition is his only guide.