200 new high quality test boreholes, made for the design of the new High Speed Underground Rail connection in Firenze, has allowed a critical review of about 1,500 lithostratigraphic logs available for the Firenze area. This has led to a new and a more accurate definition of the geometry of the lithological bodies in the Firenze subsurface as well as better understanding of the recent geological evolution of the area. The recent geological history is primarily controlled by both the Apennine (NW-SE) Fiesole master-fault which rose up to about 1,000 m and by the anti-Apennine (SW-NE) transfer-faults, bounding the Firenze structural high to the NW and SE. Their vertical displacement is assessed to be a few hundred meters. The sedimentary and erosion processes were driven by the fault movements and by climatic changes. As a result of the combination of these factors three sedimentary phases prevailed in the area. They have been interpreted in terms of UBSU as three Synthems: Synthem of the Firenze-Prato-Pistoia basin, corresponding to the lacustrine phase; Synthem of Firenze, corresponding to the deposits of the late Pliocene uplift of the Firenze area; Synthem of the Arno, corresponding to the recent deposits of the Arno river and its tributaries. Moreover, where possible, the Synthems have been subdivided into sub-units.