International audienceIn the public domain, the artistic expression embodies different forms which go from the most ›intellectual‹ and respondent to what one can see in the ›world of art‹ (installations, performances, videos etc.) to the most popular or alternative ones, like graffiti and flash mobs. 1 They all stem from esthetical, political and ethical projects. It would be impossible to account for all of them in the urban space, unless we follow Paul Ardenne's idea of contextual art, that is of unauthorised as well as legitimate actions implying the physical valorisation of the public sphere: »With ›contextual‹ art we mean all the forms of art which differ from the work of art as it is understood traditionally, such as: militant and com...