Based on Giorgio Vasari and on a certain amount of documentary evidence, we can confidently state that Niccolò Tribolo was the artist responsible for designing and beginning the construction of the grotto. Devised to celebrate Cosimo I and Eleonora, with an allusion to the myth of Egeria and Numa Pompilius, Tribolo’s design may well have already included the presence of animals, possibly to accompany an Orpheus. A statue of Orpheus was certainly present in the 17th century, and I propose to identify it here with a late 16th century marble work now standing in the Boboli Garden. After Tribolo’s death, building work largely continued to comply with his design thanks to his assistants, led by Davide Fortini, with the addition of Vasari and of Bartolomeo Ammannati. In 1565, however, the iconographical programme changed, with the introduction of bronze birds by Giambologna and Ammannati assisted, according to hitherto unpublished documentary evidence, by Giovan Battista Del Tadda; while the stone animals, carved chiefly by experts who trained with Tribolo, such as Francesco Ferrucci Del Tadda and Antonio Lorenzi, did not appear on the scene until 1580-95.