Pope Pius IV, born Giovanni Angelo Medici, was head of the Catholic Church and ruler of the Papal States from 25 December 1559 to his death, in December 1565. Born in Milan, his family considered itself a branch of the House of Medici and used the same coat of arms. Although modern historians have found no proof of this connection, the Medici of Florence recognized the claims of the Medici of Milan in the early 16th century.

The House of Medici was an Italian banking family and political dynasty that first consolidated power in the Republic of Florence under Cosimo de' Medici, during the first half of the 15th century. The family originated in the Mugello region of Tuscany, and prospered gradually until it was able to fund the Medici Bank. This bank was the largest in Europe during the 15th century and facilitated the Medicis' rise to political power in Florence, although they officially remained citizens rather than monarchs until the 16th century.