The Museum, accessible from the corridor of the sacristy in the left transept of the Basilica, presents the Treasury of the Basilica, which was reconstituted after the sacking of 846 and the sack of Rome in 1527. Despite further losses during the Napoleonic period following the Treaty of Tolentino in 1797, what remains, enriched by works of art from both the old and the new Basilica, nevertheless constitutes a collection of extraordinary importance.