I'm an art historian, so I'm pretty expert on this.

Most (but not all!) Paris museums allow photography if it's with a hand-held camera and without a flash. Many museums elsewhere have this same requirement. It used to be much more liberal; someone mentioned the Guggenheim. Years ago the Guggenheim let you photograph anything, even work from loan exhibits. No more! Two guards nearly got into a fight trying to determine who had the right to yell at me not to photograph. The Whitney recently banned photography. It's really pissing me off.

Anyhow, Paris is better. But you can't photograph at the Marmottan and a few other places, and you can't photograph temporary exhibitions.