Urban demographic transition can be defined as the historical period in which the population growth of cities structurally changes the settlement of territories. During this period, the urban population rate, which was previously very low, less than one-tenth of the population from the origins of humanity at least until the beginning of the 19th century, grew to be more than half of the population and sometimes much more. On a global average, the urban demographic transition was complete by 2008. Yet this transition is a double surprise in the history of the earth. The authors of the past had by no means envisaged it, and it is unprecedented in the history of the settlement of our planet. It will therefore be necessary to explain the unforeseen process that led to it. But it is essential to understand that urbanisation is deployed in a very differing way according to territories and countries. Finally, it is important to examine the period of post-transition, which concerns all countries whose urbanisation rate exceeds half the total population.