A map showing the location of the Metropolitan Areas in Mexico.
Metropolitan areas in Mexico have been traditionally defined as the group of municipalities that heavily interact with each other, usually around a core city.
In 2004, a joint effort between CONAPO, INEGI and the Ministry of Social Development (SEDESOL) agreed to define metropolitan areas as one of the following:
the group of two or more municipalities, in which a city with a population of at least 50,000 is located whose urban area extends over the limit of the municipality that originally contained the core city incorporating either physically or, under its area of direct influence, other adjacent predominantly urban municipalities, all of which either have a high degree of social and economic integration or are relevant for urban politics and administration; or
a single municipality, in which a city of a population of at least one million is located and fully contained (that is, it does not transcend the limits of a single municipality); or
a city with a population of at least 250,000 that forms a conurbation with other cities in the United States.
Northwestern and southeastern states are divided into a small number of large municipalities whereas central states are divided into a large number of smaller municipalities. As such, metropolitan areas in the northwest usually do not extend over more than one municipality (and figures usually report population for the entire municipality) whereas metropolitan areas in the center extend over many municipalities.
A megalopolis is defined as a long chain of continuous metropolitan areas, or territories that are relatively integrated amongst each other, a clear example of which is the Northeast Megalopolis in the United States. In 1996, the Programa General de Desarollo Urbano del Distrito Federal first proposed this concept to refer to the Mexico City megalopolis, or "Megalopolis of central Mexico", which was later expanded by PROAIRE, a metropolitan commission on the environment.
A megalopolis, is known in Spanish as a corona regional de ciudades ("regional ring of cities"). The Megalopolis of central Mexico was defined to be integrated by the metropolitan areas of Mexico City, Puebla, Cuernavaca, Toluca and Pachuca, which may also conform complex subregional rings themselves (i.e. Greater Puebla conforming a regional ring with Atlixco, San Martín Texmelucan, Tlaxcala and Apizaco).