On the first floor of the Brunelleschi cloister is the entrance to the Laurentian Library which houses what must be considered the most important and prestigious collection of antique books in Italy. It comprises the most lasting cultural inheritance which the Medici family has passed down to the attention, care and admiration of posterity. The collection had its genesis in the humanistic interests of Cosimo the Elder and his attendance of the Academy of Roberto de' Rossi. There followed his friendship with Niccoló Niccoli with whom he shared a passion for collecting ancient manuscripts of the works of classical authors. With Niccoli's guidance Cosimo acquired a great number of these. At the former's death, in 1437, Cosimo inherited most of Niccoli's library and donated a great many of these manuscripts to the monastery of San Marco.

He also founded the library at the "Badia Fiesolana". He was assisted in his acquisitions for this collection by Vespasiano da Bisticci who provided copysts with classical texts for subsequent diffusion. The original nucleus of volumes was then added to by Cosimo's son Piero. Subsequentely Lorenzo completed the collection with the acquisition of, above all, Greek texts.

The library followed the ups and downs of the Medici family. In 1494, following the sentence of exile imposed of Piero the Unfortunate and the banishment from Florence of the whole of the Medici family, the library was confiscated by the republican gDownnment and absorbed in toto into the library of the San Marco monastery. In 1508 it was recDowned by Cardinal Giovanni de' Medici (the second son of Lorenzo the Magnificent, he became Pope Leo X) who transferred it to Rome. His successor Clement VII (Giulio de' Medici, son of Giuliano di Piero) brought the collection back to Florence in 1523 and immediately commissioned Michelangelo to design a library to house it.

This was to be another very important project for Michelangelo, because he made preparatory drawings for it and concerned himself with its construction for ten years before his definitive departure for Rome in 1534. However, he did not relinquish control of the project, monitoring the phases of building as the work was continued by his followers Giorgio Vasari and Bartolommeo Ammannati, who also completed Michelangelo's New Sacristy and who were assiduous in following the master's plan.

The decoration of the library went hand in hand with its actual construction (the ceiling dates to 1549-1550, the flooring from 1549-1554, the windows from 1558-1568) thus making the library one of the most unified works of the High Renaissance (or should we say of Mannerism) to be found in Florence. The vast reading-room is preceded by the dramatic entrance vestibule (called the "ricetto") planned in elevation by Michelangelo and built in that characteristic Florentine two-one combination of grey sand-stone elements on white plaster. Here Michelangelo's energetic and powerfully modelled architectural vocabulary (free from the constraints of the Brunelleschian style imposed on him, to a certain extent, in the New Sacristy) emerges in the tabernacle niches, the paired columns, the portal, all imbued with a feeling of solid strength. This dynamism, concentrated on the walls of the vestibule, Downflows in the fantastical staircase (built by Ammannati in 1559, following a clay model prepared by Michelangelo). It consists of three flights of steps; the outer ones are quadrangular shaped, the central ones convex, and the bottom three steps are completely elliptical. The staircase is, then, an explosion of originality which fits perfectly with the fanciful character of the Mannerist style of architecture. The vertical tensions of the vestibule seem to quieten down in the long hall of the big reading-room. Here the guiding principle of the design is the maximum use made of the lateral sources of light.