Considered by some to be one of the grandest churches in Rome, this minor basilica is dedicated to Saint Ambrose (Ambrosius) and Saint Carlos Borromeo, the patron saints of Milan. The phrase "When in Rome do as the Roman do," has long been attributed to Saint Ambrose, while serving as the bishop in Milan. Unlike Roman customs, in Milan, the church congregation did not fast on Saturdays. In response to a visiting saint on how to best handle the different customs, Ambrose replied, "When I go to Rome, I fast on Saturday, but here I do not. Do you also follow the custom of whatever church you attend, if you do not want to give or receive scandal?" Later reiterations of the quote boiled it down to the phrase we know today. 

The impressive interior of the church contains many large canvas paintings, statues, painted ceilings, as well as countless ornate details. Of the many beautiful churches in Rome, this one has no admission fees or line ups.