While you can visit most museums and tourist attractions in Italy without a reservation, there are a few that you will want to book in advance to avoid long waits and some where reservations are mandatory.
If you purchase tickets online you must be careful to avoid scams and resellers, which may charge you much more than the official site. This is the case with two sites called “uffizi.com” and “uffizi.org” which look very official but are not—the official site is uffizi.it and for tickets they will redirect you to B-ticket.
The following is a list of some important Italian museums and cultural sites. If you decide to visit the ones in the “most popular” section, purchase your ticket in advance at the suggested website.

Most popular museums:

• Borghese Gallery, Rome. Online tickets: http://www.galleriaborghese.it/en/home_en/borghese-gallery-and-museum
• Cappella degli Scrovegni (Giotto Frescoes), Padua. Online tickets (must be purchased at least 24 hours in advance): http://cappelladegliscrovegni.vivaticket.it/?Language=ENG
• Cenacolo (Leonardo da Vinci’s Last Supper), Milan. Online tickets: http://www.vivaticket.it/index.php?nvpg[evento]&id_show=63954&idt=744
• Colosseum and Roman Forum, Rome. Online tickets: http://www.coopculture.it/en/colosseo-e-shop.cfm
• Leaning Tower of Pisa. Online tickets (not earlier than 20 days in advance): http://www.opapisa.it/en/
• Uffizi, Florence. Online tickets: http://www.b-ticket.com/b-ticket/uffizi/info_venue.aspx
• Vatican Museums, Rome. Online ticket: http://biglietteriamusei.vatican.va/musei/tickets/do?action=booking&codiceLivelloVisita=9&step=1

Other museums:

• Piero della Francesca’s frescoes, Arezzo.
• Basilica papale e Sacro convento di San Francesco d’Assisi, Assisi.
• Santa Giulia Museum Complex, Brescia.
• Museo Schifanoia, Ferrara.
• Galleria dell’Accademia, Florence.
• Museo nazionale del Bargello, Florence.
• Palazzo Ducale, Mantua.
• Pinacoteca Ambrosiana, Milan.
• Museo del Novecento (Museum of the 20th Century), Milan.
• Museo Poldi Pezzoli, Milan.
• Triennale, Milan.
• Galleria Estense, Modena. [Do not buy tickets from “galleriaestense.org”]
• Museo Ferrari, Maranello (Modena).
• Museo di Capodimonte, Naples.
• National Archeological Museum, Naples.
• Archeological site of Pompeii, Pompeii (Naples).
• National Gallery, Parma.
• Certosa di Pavia, Pavia.
• National Gallery of Umbria, Perugia.
• La Galleria Nazionale, Roma.
• MAXXI (National Museum of XXI Century Arts), Rome.
• National Gallery of Ancient Art (Palazzo Barberini), Rome.
• National Museum of Musical Instruments, Rome.
• Hadrian’s Villa, Tivoli (Rome).
• MART (Museum of Modern and Contemporary Art), Rovereto.
• Pinacoteca nazionale, Siena.
• Museo egizio, Turin.
• Palazzo Ducale di Urbino, National Gallery of Marche, Urbino.
• La Biennale di Venezia, Venice.
• Doge’s Palace, Venice.
• Gallerie dell’Accademia, Venice.
• Peggy Guggenheim Collection, Venice.