This paper focuses on the following question: what, if anything, makes contemporary world cities different from each other? Starting from some sessions of Research Committee 21 (Urban and Regional Development) of I.S.A. in Durban, the author suggests that a key-feature to distinguishing world cities is the acceleration of public insecurity and a deteriorating rule of law, as well the resultant social, spatial, and economic fragmentation these changes engender. So, planners, architects, and sociologists must be prepared to think about how to make or keep cities secure, without resorting only to “privatized” efforts to guarantee security. A possible solution is the creation of public spaces open to all classes and cultures, emphasizing social and economic integration. Moreover, policies for balanced urban employment patterns and alternative land uses are necessary to prevent social and spatial polarization of urban life, and compartmentalization of space and economy into high and low-end activities.