Whether you are Roman Catholic or not, a visit to the Vatican is strongly recommended for all visitors to Rome Italy. Separate queues exist for St. Peter's Cathedral, the Cathedral Dome and the Vatican Museum (which includes the Sistine Chapel). Consequently, it is a good ideal to prioritize what to you want to see before you arrive. There are no entry charges to visit St. Peter's, but admission charges are applied at the museum and to climb the dome. Wait-times at each of the attractions can be lengthy.
