Born in Pietra Ligure, he studied in Rome under Adolfo Venturi and began his own career as a teacher at Milan's Accademia scientifico-letteraria in 1905.
Two years later he was chosen for the newly established chair in art history at the University of Turin.
He moved to Florence in 1914, where he established a strong friendship with Bernard Berenson, and then Rome in 1926, remaining in the latter city until the end of his teaching career in 1948 and also dying there in 1962.
His most important students included Roberto Longhi, Ernst Kitzinger, Carlo Bertelli,[1] Giovanni Carandente and Federico Zeri.
He was also director of the medieval and modern art sections of the 1929-1937 Enciclopedia Italiana and national member of the Accademia dei Lincei from 1946 onwards.