Museum Pass

No need to purchase either in advance: just get them in Rome and Paris.

And do NOT buy the Paris Pass or Omnia Card (Rome): only the Paris MUSEUM Pass and Roma Pass. And only purchase them if the math works (you'll be seeing enough of what both cover to break even on them or come close to that, anyway).

Hi sunshine, purchase your Paris Museum Pass at one of the smaller museums that you plan to visit, ie Rodin, Cluny. Great value and enables you to skip the lines at many sites. Everybody still has to go through security, of course. Have fun in Paris, my favorite city!

Last time I was in Paris I bought my museum pass at the TI in CDG airport.

Euston and Gatwick are in the UK. I can’t imagine them selling museum passes for other countries, but admittedly, I’ve never looked.

Dale I'm exhausted ignore the London locations this planning has me so overwhelmed....

When I was in Paris I bought a museum pass at the Tabac (also sells souvenirs) across from Sainte Chapelle. If you're trying to plan which are your consecutive days it's nice to use the weather report (the pass is valid for as many days as you buy, but they must be consecutive).

If planning exhausts you, don't worry about purchasing museum passes while you're home. Buy it when you arrive. It's easy, stress-free, and simple.

I went to Paris and Italy this summer and was always able to get tickets to all the museums I wanted to go to. The only one I would recommend buying in advance is The Last Supper. There was one other in Florence that was sold out, but I forgot the name of the museum. I just remembered it, Accademia, yep purchase that in advance also.

There are some other sights in Italy, especially in Rome, for which tickets need to be purchased in advance. These days, anyone with an interest in major museums or historical sights needs a current guidebook, else he may well miss out on things he deems important.

Even with a guidebook there can be unpleasant surprises. Yesterday Enric posted information about a change in ticketing procedures at Parc Guell in Barcelona. At some point next year, it will no longer be possible to buy a ticket to the Monumental Zone at the park; you'll have to arrive with a ticket already in hand. That information will probably not make it into the 2019 guidebooks to Spain.

Buy them when you are there. Ask at your hotel, they may even have them at the desk or they'll know the closest location. Don't wait in line at a major attraction to buy them. Even if they cost more than the sights you are going to see, it's worth it to me to bypass most of the lines. In Paris we wandered into many places we otherwise would have skipped.