One of the most ancient churches in Milan, it was built by St. Ambrose in 379-386, in an area where numerous martyrs of the Roman persecutions had been buried. The first name of the church was in fact Basilica Martyrum.
The current building was begun around 1080.
The basilica has two bell towers. The right one, called dei Monaci (“of the Monks”), is from the 9th century and has a severe appearance. The left and higher one dates to 1144, the last two floors added in 1889.
The interior of the basilica has the same size as the external portico. Notable is the heavily restored apse mosaic, portraying the Christ Pantokrator and dating from the early 13th century. There are mosaics of about 470 in the oratory of San Vittore in Ciel d’Oro.
The church also houses the tomb of Emperor Louis II, who died in Lombardy in 875.