A distinctive feature of Florence's skyline is the dome of the cathedral (Duomo), Santa Maria del Fiore. The building itself, located due north of the Piazza della Signoria, was begun by the sculptor Arnolfo di Cambio in 1296. Numerous local artists continued to work on it during the following century and a half. The painter Giotto designed its sturdy bell tower (campanile) in 1334. Yet, the massive octagonal cupola (1420-36) that truly dominates both the church and the city was the proud achievement of Filippo Brunelleschi, master architect and sculptor. Opposite the cathedral stands the Baptistery; the building dates from the 11th century but was believed by Florentines to be a surviving Roman monument when they commissioned for it a series of bronze doors with relief sculptures (1330; 1401-52). The third pair of these doors, by Lorenzo Ghiberti, were of such rare beauty that Michelangelo christened them the " Gates of Paradise."
The cathedral of Florence itself had been begun in the Gothic style by Arnolfo di Cambio in 1296. But in 1366 the City of Florence, following the advice of certain painters and sculptors, decided that the Gothic should no longer be used and that all new work should follow Roman forms, including an octagonal dome 42 metres in span to be built at the east end of the nave. The dome was not built until the early 15th century, when Filippo Brunelleschi, a goldsmith and sculptor, began to make statues for the cathedral. Gradually he became interested in the building itself and built some smaller parts of it. In about 1415 he prepared a design for the dome that he daringly proposed to build without the aid of formwork, which had been absolutely necessary in all previous Roman and Gothic construction. He built a 1:12 model of the dome in brick to demonstrate his method; the design was accepted and built under his supervision from 1420 to 1436.
Brunelleschi's dome consists of two layers, an inner dome spanning the diameter and a parallel outer shell to protect it from the weather and give it a more pleasing external form. Both domes are supported by 24 stone half arches, or ribs, of circular form, 2.1 metres (seven feet) thick at the base and tapering to 1.5 metres (five feet), which meet at an open stone compression ring at the top. To resist outward thrust, tie rings of stone held together with metal cramps run horizontally between the ribs. There are also tie rings of oak timbers joined by metal connectors. The spaces between the ribs and tie rings are spanned by the inner and outer shells, which are of stone for the first 7.1 metres and brick above. The entire structure was built without formwork, the circular profiles of the ribs and rings being maintained by a system of measuring wires fixed at the centres of curvature. Brunelleschi obviously understood enough about the structural behaviour of the dome to know that, if it were built in horizontal layers, it would always be stable and not require timber centring. He also designed elaborate wooden machines to move the needed building materials both vertically and horizontally. Having all but equaled the span of the Pantheon in stone, Brunelleschi was hailed as the man who "renewed Roman masonry work"; the dome was established as the paragon of built form.
A modern fašade of the cathedral, executed by Emilio de Fabris in 1867-87 in the style of the Gothic Revival, has taken the place of one which was destroyed at the end of the 16th century.
The Cathedral Museum (Museo dell'Opera del Duomo) houses famous statues from the former fašade, from the interior and from the campanile, as well as manuscripts from the cathedral's possession.
You can find more information on the Duomo in the Wikipedia and on its home page.
Recommended viewing from the collection:
The Web Gallery of Art contains 165 images of artworks from the Duomo, the Baptistry and the Cathedral Museum.