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 few metropolitan areas extend beyond the limits of one state: Greater Mexico City (Federal District, Mexico and Hidalgo), Puebla-Tlaxcala (Puebla and Tlaxcala, but excludes the city of Tlaxcala), Comarca Lagunera (Coahuila and Durango), and Tampico (Tamaulipas and Veracruz).
There are a total of fifty-six metropolitan areas of Mexico as defined by the following government bodies:The National Institute of Statistics and Geography (INEGI).
The Secretariat of Social Development (SEDESOL).
The National Population Council (CONAPO).
The United States shares a 2,000-mile (3,000 km) border with Mexico. The 2,000 miles is the most frequently crossed international border in the world, with about 250 million legal crossings every year. The distribution of population and urban population in Mexico has been changed significantly by the economic interaction between settlements in its north and the United States (U.S.). The increasing population concentration in the north of Mexico is strongly associated with the development of the maquila industries there and the eventual economic effects of North American Free Trade Agreement (NAFTA).
Metropolitan areas located at the border with the U.S. form transnational conurbations with deep economic and demographic interaction. For example, the San Diego – Tijuana metropolitan area consists of San Diego County in the U.S. and the municipalities of Tijuana, Playas de Rosarito, and Tecate in Mexico. The total population of the region has been estimated to be just over 5 million in 2009, making it by far the largest bi-national metropolitan area shared between the U.S. and Mexico. The National Population Council (CONAPO) recognizes the existence of such metropolitan areas and defines them as the municipalities that contain a city of at least 200,000 inhabitants which share processes of conurbation with cities of the U.S.
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).
The megalopolis consists of 173 municipalities (91 of the state of Mexico, 29 of the state of Puebla, 37 of the state of Tlaxcala, 16 of Morelos and 16 of Hidalgo) and the 16 boroughs of the Federal District, with an approximate total population of almost 27 million people.