Following the completion of the construction of new St. Peter's in the first quarter of the seventeenth century, a series of monumental altarpieces was commissioned to decorate its altars. The leading artists of the day contributed to the project - among them Algardi, Bernini, Cortona, Domenichino, Guercino, Lanfranco, Poussin, Sacchi, Vouet, and Valentin - and the works they produced include many of the most celebrated masterpieces of the Roman Baroque. Here for the first time the altarpieces of St. Peter's are considered collectively, within the liturgical and artistic program of the building as a whole. The book offers a comprehensive analysis of this critical chapter in the history of Italian Baroque art and provides insight into the mechanisms, motives, and meanings of papal patronage in the premier church of Catholicism.