The purpose of this paper is to explain sudden urban growth, a global phenomenon of astonishing force that presents one of the greatest challenges facing humanity today. The forces that shape urban growth are many and varied. Such forces produce an array of histories of urbanization ranging from continuity to various catastrophic shifts along the population trajectory. It is seen that the realized history of urbanization explicitly depends on the prevailing characteristics of individuals and of the existing technology. Hence, in principle, given a knowledge of these characteristics (and given some other information explained in the text), it would be possible to predict the path toward urbanization taken by simple worlds such as those considered here, Different outcomes would then be attributed to and explained by different behavioural and technological circumstances.