I was just at the Great Synagogue in Rome several weeks ago (for the fourth time). You enter the synagogue area through a security gate on the back corner of the property, across from Portico d'Ottavia, then down some steps into the museum on the lower level of the synagogue. You then buy a ticket for the museum (which is quite well done, and very informative with multiple displays in English, and multiple audio-visual screens throughout the some ten or so rooms). The cost of entry to the museum also covers the cost of the tour. When you buy your ticket, ask when the next tour will be in English (I'm guessing every hour or so), then listen for the guide who will walk through the rooms announcing "Tour in English!" We scheduled ourselves to get to the synagogue when it opened, and the first English tour was about 15 minutes after opening time, so we visited the museum after the tour. Re. Venice: Yes, you can hire a guide to show you around the Venetian Ghetto, but there is also a nice, smaller museum in the Ghetto, which you can visit on your own, and they offer a tour of three beautiful, historic synagogues in the Ghetto, led by their own guides. When you go to the museum, ask when the next tour will be in English. You cannot visit these synagogues without a tour group (or guide). Two are on the upper floors of buildings close by, and one is larger, in a self-contained building, and still in regular use. They actually vary which third synagogue to visit; one is used by the community during the summer, and one is used during the winter. I believe that the tour goes to visit whichever one is in use during the season of your visit. Again: my experience, which is of having visited both the Great Synagogue in Rome several times, and the Jewish Museum and synagogues of Venice several times, leads me to understand that purchasing tickets to visit the synagogues ahead of times is not necessary, and likely not possible. Do be aware of Sabbath times (close early on Fridays and closed on Saturdays) and Jewish holidays, when none of these will be open to the public. Oh, and finally: There is a lovely, historic synagogue in Florence, too, within easy walking distance of the historic center (10-15 minute walk from either Santa Croce or the Bargello).