Dealing with both urban dynamics and urban sustainability can appear as a paradox insofar as these terms seem radically opposed. But, obviously, thanks to the dynamics they created and from which they gained advantage, cities have been sustainable since they first appeared. This paper will demonstrate that it is no use trying to oppose dynamics and sustainability inasmuch as sustainability appears as the contemporary form of dynamics. Taking this major evolution in urban history into consideration will first enlighten the present context which promotes such innovations, then define the new kind of cities that are about to be produce. Last, it will focus on an epistemological reflection upon the social sciences and their ability to provide tools to analyse and explain what currently occurs in cities.