The evolution of cities, whose pattern is all but regular, is sometimes marked by rare events that may disrupt the normal development. Throughout history, cities grow and shrink, flourish and perish, in a slow and somehow predictable way, but sometimes they suffer sudden and unexpected changes. Those breakages that profoundly modify their urban schemes and land-uses, their identity and their economic and social activities, transform them into entirely new cities or radically convert large portion of space. Whatever the outcome of a disaster could be, although it may result in a pattern of positive development, we must prevent the loss of human lives, the suffering, the loss and damages that accompanies every catastrophe. The main task of human action must therefore be risk mitigation, bearing in mind that risk is always present and the resources to mitigate it are often scarce. Without going into the detail of any particular event and with methodological intent, the paper will try to investigate how we can better understand risk and how planning may influence the mitigation of risk.