The Medici Chapels form part of a monumental complex developed over almost two centuries in close connection with the adjoining church of S. Lorenzo, considered the "official" church of the Medici, who chose it at the time they lived in the neighbouring palace of Via Larga (now MediciRiccardi Palace, see the related section). The decision to build their mausoleom in this church dates back to the 14th century (Giovanni di Bicci and his wife Piccarda were for instance buried in the Old Sacristy, built on a project of Brunelleschi).
The project of building a proper family mausoleum was conceived in 1520, when Michelangelo started working at the New Sacristy upon request of Cardinal Giulio de'Medici, the future Pope Clemens Vll, who expressed the desire to erect a mausoleum on behalf of some members of his family: Lorenzo the Magnificent and his brother Giuliano, Lorenzo Duke of Urbino and Giuliano, Duke of Nemours.
After completing the architectural works in 1524, Michelangelo worked until 1533 on the sculptures that wonld have decorated the walls and the very original sarcophagi. The only ones actually completed were the statues of Lorenzo Duke of Urbino and of Giuliano Duke of Nemours, in addition to the allegories of Day and Nitgh, Dawn and Dusk, and the groop representing the Madonna with child flanked by Saints Cosma and Damiano (protectors of the Medici), executed respectively by Montorsoli and Baccio da Montelupo. both pupils of Buonarroti.

The solid and articulated architectural structure and the strength of Michelangelo's sculptures clearly reflect a complex symbolism that offers an interpretation of Human Life where active and contemplative life interact to free the soul after death, following a philosophical concept that was closely linked with Michelangelo's spirituality.

Numerous drawings by Michelangelo were found in a small space beneath the apse. They had been drawn, as often occurred, by the artist and may be related to the statues and architecture of the Sacristy.

The Chapel of the Princes

This Chapel is yet another grandiose and pompous mausoleum erected between 1604 and 1640 by the architect Matteo Nigetti to the desigus of Giovanni de' Medici, a member of the family who practised architecture in a semi-professional manner. The Mausoleum was conceived to celebrate, with its large dome and lavish interior decorated with marbles, the power of the Medici dynasty, which had safely been ruling Florence for several centuries. The octagonal room designed to receive the bodies of the grand dukes is in fact almost entirely decorated with semi-precious stones and marbles. The grand ducal sarcophagi are completed with bronze statues and inserted in niches. The inlay of the semiprecious stones, partially executed by the extremely skilled workers employed in the laboratories of the Opificio delle Pietre dure (see the related section) took several centuries to be completed due to the difficulty of finding these materials, available only at a very high cost.

The dome should have originally had an internal coating of lapis lazuli but was left incomplete at the end of the Medici period and frescoed in 1828 by Pietro Benvenuti with scenes of the New and Old Testament at command of the reigning Lorraine family.