The church is known as San Carlino because it is so tiny: its dimensions are no greater than the area taken up by just one of the great pillars that hold up St. Peter's dome. A masterpiece by Borromini, the greatest of the baroque architects, the work of this church was started in 1638 and followed closely by Borromini himself for it was a creation in a new and revolutionary structural language.

Unfortunately the building was incomplete when he died. The lines of the façade echo the troubled spirit of this extraordinary personality. Tormented in life as he was in art, made worse by his rivalry with Bernini on the Roman art scene, Borromini committed suicide, which, tragically, prevented his burial in the small chapel he had prepared inside this church.