Cosimo I de' Medici (12 June 1519 – 21 April 1574) was the second Duke of Florence from 1537 until 1569, when he became the first Grand Duke of Tuscany, a title he held until his death.

Cosimo was born in Florence on 12 June 1519, the son of the famous condottiere Ludovico de' Medici (known as Giovanni dalle Bande Nere) and his wife Maria Salviati. He was the grandson of Caterina Sforza, the Countess of Forlì and Lady of Imola. Cosimo came to power in 1537 at age 17, just after the 26-year-old Duke of Florence, Alessandro de' Medici, was assassinated. Cosimo was from a different branch of the Medici family, descended from Giovanni de' Medici il Popolano, the great-grandson of Giovanni di Bicci de' Medici, founder of the Medici Bank. It was necessary to search for a successor outside of the "senior" branch of the Medici family descended from Cosimo di Giovanni de' Medici, since the only male child of Alessandro, the last lineal descendant of the senior branch, was born out-of-wedlock and was only four years' old at the time of his father's death. Up to the time of his accession, Cosimo had lived only in Mugello (the ancestral homeland of the Medici family) and was almost unknown in Florence. However, many of the influential men in the city favoured him as the new duke. Several hoped to rule through him, thereby enriching themselves at the state's expense. However, as the Florentine literatus Benedetto Varchi famously put it, "The innkeeper's reckoning was different from the glutton's." Cosimo proved strong-willed, astute and ambitious and soon rejected the clause he had signed that entrusted much of the power of the Florentine duchy to a Council of Forty-Eight.

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