This page lists the 50 largest metropolitan areas of the world, listed by population. This is a controversial issue rather difficult to quantify, as unlike national subdivisions, there are no defined borders for such an area. Many of the figures for individual cities are highly controversial, and in some cases numbers which differ by millions are given by different sources. In some countries there are official definitions, but they are not consistent on a global basis, nor even necessarily within countries. In addition, the underlying structure of different metropolitan areas varies: comparing a highly centralised city with an agglomeration of smaller settlements may not be very meaningful. Commuting patterns and other aspects of urban behaviour which underlie the metro area concept differ