Built by Barnabite monks in the 9th century on the ruins of  the praetorium, where tradition has it that the martyr St. Alexander was held prisoner, the church has a centralised plan with a cupola that serves as a presbytery. The building is widely held to be the work of Alessi, who had just returned from Genoa at the time and was active in Milan. The façade is decorated with bas-reliefs and flanked by two bell towers. The old square where the church stands is one of the very few spaces in Milan that have remained untainted by post-war reconstruction. The baroque interior is decorated with a series of valuable paintings in the Lombard tradition of the 17th and 18th centuries: the Assumption by Camillo Procaccini in the last chapel in the right nave, the Nativity Scene, considered one of the artist’s best works, in the chapel at the end of the right nave, and the Crucifixion in the first chapel on the left. Other paintings of note include The Flagellation by Daniele Crespi, and the painting of the Hosanna in the first chapel on the right.Milanese families contributed to the building of the church, obtaining patronage of the many chapels through their large bequests.
The remains of Saint Alexander are preserved in the chapel dedicated to Saint Pancras.
According to an hypothesis exhibited on the tower, the praetorium was called “di Zebedia” after a commanding judge.
Wooden decorations and frescoed vaults are typical of the Barnabite style.
The Church is rich with pictorial works of the 1600s and frescoes realized by Federico Bianchi, Filippo Abbiati and many others. The main altar (1741) is the work of G.B. Riccardi.