This paper presents the comparative analysis of the National Museum of Modern Art, in the Pompidou Centre, Paris, designed by Richard Rogers and Renzo Piano (1972–77), and the Tate Modern art gallery, London, the conversion of an industrial building by Swiss practice Herzog & de Meuron (1995–2000). The two museums share a set of conspicuous similarities so that their parallel investigation seems self-evident. Both are large-scale national museums of modern art, extending over two floors, in buildings that constitute urban landmarks and are often seen as examples of the museum as a box [1a–b]. Their ground floors are conceived as a space you walk through, as a ‘piazza’; their spatial organisation is modular and flexible; their visual construction, punctuated by powerful views to the city. Moreover, they are guided by similar spatial ideas and share common fundamental morphological properties. Interestingly, their affinities extend to their collections – both begin with the turn of the twentieth century and extend to the twenty-first century; and their curatorial practices – as, for instance, the practice of reprogramming the galleries on a regular basis. But the experience of visiting the two museums is entirely different and each appears to have its own idiosyncratic spatial character, quite distinct from the other (described metaphorically by the museum designers as the museum as a city in the case of the Pompidou and as a machine for showing art in the case of Tate Modern). So, could these obvious similarities hide critical differences between the two museums?