Certain aspects of urban transformation can give rise to psychological and social vulnerability. The increase in residential mobility and territorial fluidity underlying the growth of the new global economy has led to new global and regional hierarchies. In addition, the polarization in the distribution of profit and the composition of work, evident in all the largest cities, places under discussion the notion of rich countries and rich cities, thus producing a new geography of centrality and marginality. The new urban hierarchies require competitive strategies for the global market, entrusted to urban marketing, for which reason the postmodern city entrusts its own reputation to the capacity to activate pull factors in order to be competitive in the urban hierarchization. The hypothesis, confirmed throughout Europe, of a mobility towards large cities leads us to point out certain aspects which we believe to have repercussions on situations of vulnerability. We refer to the link between urban fluidity and growth in accessibility to the city. Considering the scenarios characterizing European cities, and especially Italian cities, we identify new ways of interpreting the phenomenon of vulnerability that we believe can result from the very mechanisms of urban accessibility.