Much has been written about the institutional, economic and cultural politics of Cosimo I de’ Medici (1519-1574) during the nearly four decades of his rule. However, only in recent years have scholars begun to assess Cosimo I’s more personal sphere, largely thanks to work on the correspondence in the Medici Grand Ducal Archive (Mediceo del Principato), housed at the Archivio di Stato in Florence. Thousands of letters written by and about the duke paint portraits as intimate and revelatory as those painted by Agnolo Bronzino. Details about his personality and his relationship with family members are constantly emerging. These letters also record his physical maladies and psychological distress, his cynicism, his humor and his compassion. They speak of his aesthetic tenets, intellectual curiosity, military values, and culinary predilections. Letters address his obsession with his enemies, his conflicting relationships with foreign regents, and his dynastic ambitions. Most importantly, they shed light on the intricate mechanism of court culture, which saw Cosimo I at the epicenter of his rule.