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 greatly around the world. Thus, all population figures for specific metro areas should be treated as interpretations rather than hard facts, and rankings and comparisons of metros should be treated with great caution. (~from Wikipedia)
Note: The information (and numbering) below is in flux, and information now might vary from other pages.