The Museum of Modern Art (Musée d’Art Moderne de Paris) displays much of Paris’s vast collection of 20th-century art. The permanent collection, which is usually free and seen without tickets or time slot reservations, covers all periods from Fauvism to contemporary art. The pre-1940s collection is particularly interesting.
The Modern Art Museum uses half of the Palais de Tokyo — a beautiful Art Deco building, erected for the Exposition Internationale des Arts et des Techniques in Paris in 1937. The building itself, including the Art Deco metopes and bronze doors, is worth seeing too and is accessible for free.
The other half of the building is a national gallery and claims to be “Europe’s largest center for contemporary creation”. It often refers to itself simply as the Palais de Tokyo. (The name came from the previous street address rather than the Japanese capital city.)