The Medici Chapels (Cappelle medicee) are two structures at the Basilica of San Lorenzo, Florence, Italy, dating from the 16th and 17th centuries, and built as extensions to Brunelleschi's 15th-century church, with the purpose of celebrating the Medici family, patrons of the church and Grand Dukes of Tuscany. The Sagrestia Nuova ("New Sacristy") was designed by Michelangelo. The larger Cappella dei Principi ("Chapel of the Princes"), though proposed in the 16th century, was not begun until the early 17th century, its design being a collaboration between the family and architects.
These are not to be confused with the Magi Chapel in the Palazzo Medici Riccardi, then the main Medici home. This houses a famous cycle of frescoes by Benozzo Gozzoli, painted around 1459.
The Sagrestia Nuova was intended by Cardinal Giulio de' Medici and his cousin Pope Leo X as a mausoleum or mortuary chapel for members of the Medici family. It balances Brunelleschi's Sagrestia Vecchia, the "Old Sacristy" nestled between the left transept of San Lorenzo, with which it consciously competes, and shares its format of a cubical space surmounted by a dome, of gray pietra serena and whitewashed walls. It was the first essay in architecture (1519–24)  of Michelangelo, who also designed its monuments dedicated to certain members of the Medici family, with sculptural figures of the four times of day that were destined to influence sculptural figures reclining on architraves for many generations to come. The Sagrestia Nuova was entered by a discreet entrance in a corner of San Lorenzo's right transept, now closed.
Though it was vaulted over by 1524, the ambitious projects of its sculpture and the intervention of events, such as the temporary exile of the Medici (1527), the death of Giulio, now Pope Clement VII and the permanent departure of Michelangelo for Rome in 1534, meant that Michelangelo never finished it. Though most of the statues had been carved by the time of Michelangelo's departure, they had not been put in place, being left in disarray across the chapel, and later installed by Niccolò Tribolo in 1545. By order of Cosimo I, Giorgio Vasari and Bartolomeo Ammannati finished the work by 1555.
There were intended to be four Medici tombs, but those of Lorenzo the Magnificent and his brother Giuliano (modestly buried beneath the altar at the entrance wall) were never begun. The result is that the two magnificent existing tombs are those of comparatively insignificant Medici: Lorenzo di Piero, Duke of Urbino and Giuliano di Lorenzo, Duke of Nemours. Their architectural components are similar; their sculptures offer contrast. On an unfinished wall, Michelangelo's Madonna and Child flanked by the Medici patron saints Cosmas and Damian, executed by Giovanni Angelo Montorsoli and Raffaello da Montelupo respectively, to Michelangelo's models, are set over their plain rectangular tomb.
In a statement in the Michelangelo's biography published in 1553 by his disciple, Ascanio Condivi, and largely based on Michelangelo own recollections, Condivi gives the following description: "The statues are four in number, placed in a sacristy . . . the sarcophagi are placed before the side walls, and on the lids of each there recline two big figures, larger than life, to wit, a man and a woman; they signify Day and Night and, in conjunction, Time which devours all things… And in order to signify Time he planned to make a mouse, having left a bit of marble upon the work (which [plan] he subsequently did not carry out because he was prevented by circumstances), because this little animal ceaselessly gnaws and consumes just as time devours everything”.  In 1976,a concealed corridor with drawings on the walls by Michelangelo was discovered under the New Sacristy.
Day is a marble sculpture by Michelangelo, datable to 1526–1531. It is paired with Night on the tomb of Giuliano de' Medici in the chapel.
Night is a sculpture in marble (155x150 cm, maximum length 194 cm diagonally) by Michelangelo Buonarroti. Dating from 1526–1531, it is part of the decoration of the New Sacristy and part of an allegory of the four parts of day. It is situated on the left of the sarcophagus of the tomb of Giuliano di Lorenzo de' Medici, Duke of Nemours.
Along with his Dawn, Michelangelo drew from the ancient Sleeping Ariadne for his sculpture's pose.
In his poem "L'Idéal" from Les Fleurs du Mal, French Romantic poet Charles Baudelaire references the statue:
In his Life of Michelangelo, Giorgio Vasari quotes an epigram by Giovanni Strozzi, written, perhaps in 1544, in praise of Michelangelo's Night:
Michelangelo responded in 1545–46 with another epigram, entitled "Risposta del Buonarroto" (Buonarroto's response). Speaking in the voice of the statue, it may contain a scathing critique of Cosimo I de' Medici's governance, according to Kenneth Gross:
Dawn is a sculpture by Michelangelo, executed for the chapel. It is 6 feet and 8 inches in length. Along with his Night, Michelangelo drew from the ancient Sleeping Ariadne for his sculpture's pose. This was in turn influential on Benvenuto Cellini's Diana of Fontainebleau.
Dusk is a marble sculpture by Michelangelo, datable to 1524–1534. It is paired with Dawn on the tomb of Lorenzo II de' Medici.
The creation of Dusk started simultaneously with the resuming of work at the New Sacristy of Florence, during 1524, after Clement VII's election to the papal throne. The sculpture's termination date remains unknown; however, the works were interrupted during the Siege of Florence and resumed in 1531. The work remained visibly unfinished in 1534, the year that Michelangelo definitively left Florence.
Dusk, or Sunset, is personified as man and is stretched out and nude, like the other statues in the series. It was modelled, perhaps, after the mountain and river gods at the Arch of Septimius Severus in Rome.
If its pair, Dawn is in the act of awaking, Dusk is falling asleep. The statue lies down with one leg crossing the other, for greater compositional dynamism, one arm resting on his thigh to hold back a falling cloth. The other arm is bent to support the figure. The statue's face is bearded, with a thoughtful, downward gaze.
Among the various iconographic meanings proposed, the statue is seen as an emblem of the phlegmatic temperament or of the elements of water or earth. Michelangelo's study for Dusk is known for exemplifying his style of striking, unfinished drawings.
The lantern at the top of the New Sacristy is made out of marble and has an "...unusual polyhedron mounted on the peak of the conical roof". The orb that is on top of the lantern has seventy-two facets and is about two feet in diameter. The orb and cross, that is on top of the orb, are traditional symbols of the Roman and Christian power, and recalls the similar orbs on central dome plan churches like St. Maria del Fiore and St. Peter's. But because it is on a private mausoleum, the Medici family is promoting their own personal power with the orb and cross, laurel wreath and lion heads, which are all symbols of status and power.
The lantern that holds up the orb helps to accentuate the height and size of the chapel, which is fairly small. The lantern is a bit less than seven meters tall and "...is equal to the height of the dome it surmounts". The lantern metaphorically expresses the themes of death and resurrection. The lantern is where the soul could escape and go from "...death to the afterlife".
The octagonal Cappella dei Principi surmounted by a tall dome, 59 m. high, is the distinguishing feature of San Lorenzo when seen from a distance. It is on the same axis as the nave and chancel to which it provides the equivalent of an apsidal chapel. Its entrance is from the exterior, in Piazza Madonna degli Aldobrandini, and through the low vaulted crypt planned by Bernardo Buontalenti before plans for the chapel above were made.
The opulent Cappella dei Principi, an idea formulated by Cosimo I, was put into effect by Ferdinand I de' Medici. It was designed by Matteo Nigetti, following some sketches tendered to an informal competition of 1602 by Don Giovanni de' Medici, the natural son of Cosimo I, Grand Duke of Tuscany, which were altered in the execution by the aged Buontalenti. A true expression of court art, it was the result of collaboration among designers and patrons.
For the execution of its astonishing revetment of marbles inlaid with colored marbles and semi-precious stone, the Grand Ducal hardstone workshop, the Opificio delle Pietre Dure was established. The art of commessi, as it was called in Florence, assembled jig-sawn fragments of specimen stones and porphyry to form the designs of the revetment that entirely cover the walls. The result was disapproved of by 18th- and 19th-century visitors, but has come to be appreciated for an example of the taste of its time. Six grand sarcophagi are empty; the Medici remains are interred in the crypt below. In sixteen compartments of the dado are coats-of-arms of Tuscan cities under Medici control. In the niches that were intended to hold portrait sculptures of Medici, two (Ferdinando I and Cosimo II) were executed by Pietro Tacca (1626–42).