This article are lists of metropolitan areas by population by population according to different sources. The question of which are the world's largest cities is a complex one, to which there is no single correct answer, simply because there are many different ways of defining a "city".
One concept is that of the metropolitan area, which is based on the concept of a labor market area and is typically defined as an employment core (an area with a high density of available jobs) and the surrounding areas that have strong commuting ties to the core. There is currently no generally accepted, globally consistent definition of exactly what constitutes a metropolitan area, thus making comparisons between cities in different countries especially difficult.[1]
One attempt at arriving at a consistently defined metropolitan area concept is the study by Richard Forstall, Richard Greene, and James Pick. The basic principles of their definition involve delineating the urban area as the core, then adding surrounding communities that meet two criteria: (1) Less than 35% of the resident workforce must be engaged in agriculture or fishing; and (2) At least 20% of the working residents commute to the urban core.[1] Based on their consistently defined metropolitan area criteria, they tabulate a list of the twenty largest metropolitan areas in 2003.[1][2] As population figures are interpreted and presented differently according to different methods of data collection, definitions and sources, these numbers should be viewed as approximate. Data from other sources may be equally valid but differ due to being measured according to different criteria or taken from different census years.
^Consists of two separate zona metropolitanas: the Zona Metropolitana del Valle de Mexico (ZMVM) and the Zona Metropolitana de Toluca (ZMT)
^Consists of two separate metropolitan statistical areas (MSA): the New York–Northern New Jersey Long Island MSA and the Bridgeport-Stamford-Norwalk MSA.
^There are restrictions on the movement of labor between Hong Kong and mainland China and the two areas are sometimes listed separately in other lists.