Professor Kleiner discusses the revolution in Roman architecture resulting from the widespread adoption of concrete in the late second and first centuries B.C. She contrasts what she calls innovative Roman architecture with the more traditional buildings already surveyed and documents a shift from the use of concrete for practical purposes to an exploration of its expressive possibilities. The lecture concludes with a discussion of the Sanctuary of Fortuna Primigenia at Palestrina, an impressive terraced complex that uses concrete to transform a mountain into a work of architecture, with ramps and stairs leading from one level to the next and porticoes revealing panoramic views of nature and of man-made architectural forms.