Many of the world’s biggest cities are getting bigger still. In 2017, the number of megacities --- urban areas with better than ten million people --- increased to 37 in 2017, as the Chennai urban area entered their ranks. Chennai becomes India’s fourth megacity, along with Delhi, Mumbai and Kolkota. These are among the major findings in the just released 13th annual edition of Demographia World Urban Areas, which provides population, land area and population density estimates for the 1,040 identified built-up urban areas (cities) in the world. Built-up urban areas are the physical form of the city, a definition which separates out the urban, or constructed form of the city from the rural and smaller town areas with which they form a metropolitan area or labor market (Figure 1).