Cathedral of Santa Maria Assunta is the main Roman Catholic Cathedral of Aosta. The place where today lies the city of Piazza Giovanni XXIII, was once the southern part of the Roman forum during the existence of the colony of Augusta Pretoria. Even after the fall of the Roman Empire and decline of the colony, this place has not lost its primary significance in the lives of citizens. Here, to the West of the cloister - Criptoportico, at the end of the 4th century was built the first Christian building Aosta. It was a building of impressive dimensions with a single nave ending in an apse, baptistery in the Western part and the different rooms, one of which was used as a second baptistery. The façade of the Cathedral was located a few metres from the Eastern wing of Criptoportico and was actually connected to it. The entire complex, to which later was added a few rooms, for several centuries it was used as the residence of the Bishop and clergy, and its appearance did not undergo significant changes until the late middle ages. A precious cycle of frescoes found during archaeological work in the attic of the Church also dates back to the 11th century – thanks to these murals and frescoes in the Church of Sant'orso, Aosta was considered the center Otoscope art in Europe.

In the second half of the 11th century the Western part of the Cathedral was completely rebuilt – then it consisted of two towers and overhanging the Central apse. In the 13th century, two of the five original apses were demolished and in their place erected a covered gallery and a ring-shaped corridor around choirs. Between the 15th and 16th centuries, the Cathedral on the initiative of the then Bishop was decorated with various works of art. In the upper choir loft, wooden crucifix, there are two rows of carved seats, and the floor was paved with mosaics. The main Baroque throne of the Cathedral of Santa Maria Assunta is made of black marble with colored inclusions. Two staircases lead from the choir down in the crypt of the 11th century with a small medieval columns.

The current facade of the Cathedral consists of two different parts: the atrium dates back to the 16th century, and the neoclassical pediment was erected in 1848, the year. The atrium is decorated with terracotta statues and frescoes. Near the Church, on the North side, is the cloister – a covered gallery. It was built in 1460, the year in place of a more ancient and notable in that it used different materials - stone grey "bardiglio" for pilasters, crystalline limestone for capitals and for Sandstone arches and tiles. In the centre stands the Romanesque column with Corinthian capitals.

In 1985, the year the Cathedral was a Museum introducing visitors to the local art 13-18 centuries.