Italy has many majestic cathedrals, often with spectacular works of art inside. A cathedral is a city's main church and is usually called the duomo but also can be named basilica, cattedrale or chiesa madre (mainly in the south).
There are a few rules to observe when visiting a cathedral such as no skimpy clothing, speaking quietly, and sometimes not taking photos inside. While most cathedrals do not charge admission there are a few that do.
This is not always so, however: there are places where the cathedral and the principal church are not the same (Bologna, for example); and very many places which are not the seats of bishops have a non-episcopal "duomo" and no cathedral at all. In this list, churches known as "duomo" are only included if they are, or have been, episcopal seats, as above.
There is a very small number of churches, such as that at Monza, which have such exceptional distinction or status that they are comparable in importance to cathedrals without having ever been the seats of bishops, and are commonly known in English as cathedrals. There is a separate (incomplete) list for this small group of churches.
These are my picks for top Italian cathedrals to visit.
St. Peter's Basilica (Rome): Its roots began with the first Christian emperor, Constantine, in A.D. 324. By 1400, the Roman basilica was in danger of collapsing, prompting the Renaissance popes to commission plans for the largest, most impressive, most jaw-dropping cathedral the world had ever seen. Amid the rich decor of gilt, marble, and mosaics are countless artworks, including Michelangelo's Pietà. Other sights here are a small museum of Vatican treasures and the eerie underground grottoes containing the tombs of former popes, including the most recently interred, John Paul II. An elevator ride (or a rigorous climb) up the tower to Michelangelo's glorious dome provides panoramic views of Rome.
The Duomo of Florence: Begun in the late 1200s and consecrated 140 years later, the pink, green, and white marble Duomo was a symbol of Florence's prestige and wealth. It's loaded with world-class art and is one of Italy's largest and most distinctive religious buildings. A view of its red-tiled dome, erected over a 14-year period in what was at the time a radical new design by Brunelleschi, is worth the trip to Florence. Other elements of the Duomo are Giotto's campanile (bell tower) and the octagonal baptistery (a Romanesque building with bronze doors).
The Duomo of Siena: Begun in 1196, this cathedral is one of the most beautiful and ambitious Gothic churches in Italy, with extravagant zebra-striped bands of marble. Masterpieces here include a priceless pavement of masterful mosaics, an octagonal pulpit carved by master sculptor Nicola Pisano, and the lavishly frescoed Piccolomini Library.
Basilica di San Francesco (Assisi): St. Francis, protector of small animals and birds, was long dead when construction began on this double-tiered showcase of the Franciscan brotherhood. Giotto's celebrated frescoes reached a new kind of figurative realism in Italian art around 1300, long before the masters of the Renaissance carried the technique even further. Consecrated in 1253, the cathedral is one of the highlights of Umbria and the site of many pilgrimages. It took a direct hit from the 1997 earthquakes but has miraculously made a recovery.
The Duomo of Modena: Begun in 1099, this cathedral in a city in Emilia-Romagna is one of the crowing glories of Romanesque architecture in Italy. Divided into three parts, the facade is crowned by the "Angel of Death" carrying a fleur-de-lis, and inside is filled with masterpieces of sculpture, including a rood screen that is supported by Lombard "lions."
St. Mark's Basilica (Venice): Surely the most exotic and Eastern of the Western world's churches, the onion-domed and mosaic-covered San Marco took much of its inspiration from Constantinople. Somewhere inside the mysterious candlelit cavern of the 1,000-year-old church, which began as the private chapel of the doges, are the remains of St. Mark, patron saint of Venice's ancient maritime republic.
The Duomo of Milan: It took 5 centuries to build this magnificent and ornate Gothic cathedral, the third-largest church in the world. It's marked by 135 marble spires, a stunning triangular facade, and thousands of statues flanking the massive but airy, almost fanciful exterior.
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Italy has many majestic cathedrals, often with spectacular works of art inside. A cathedral is a city's main church and is usually called the duomo but also can be named basilica, cattedrale or chiesa madre (mainly in the south). 
