The greater Mexico City metropolitan area is one of the world's largest and the largest city by population in North America, with an estimated 26 million people living in the region. It is shaped roughly like an oval of about 60 by 40km, built on the dry bed of Lake Texcoco, and surrounded on three sides by tall mountains and volcanoes such as the Ajusco, the Popocatepetl and the Ixtaccihuatl. Mexico City proper (with a population of 8.8 million as of 2010) is in the Federal District (Spanish: Distrito Federal or D.F.), a federally-administered area (that is, not part of any Mexican state) which acts as the capital of Mexico. The rest of the metropolitan area extends beyond it into Mexico State, which surrounds D.F. on three sides. According to common usage, Mexico City is the same as the Federal District, but these are arbitrary legal fictions and aren't an accurate measure of city size. A large portion of Mexico City (urban pop. approx. 20m) is in Mexico State. Much in the same way that the city of Washington (urban pop. 4,586,770) has grown far beyond the limits of the District of Columbia in the United States. Hence, the creation of each city's respective metropolitan area in the 50's and 60's. The Federal District however is where most tourists will spend the majority of their time while visiting the city.

Mexico City is divided up into 16 delegaciones, similar to the boroughs of New York, which in turn are divided into "colonias" (neighborhoods), of which there are about 250. Knowing what colonia you're going to is essential to getting around, almost all locals will know where a given colonia is (but note that there are some colonias with duplicate or very similar names). As with many very large cities, the structure is relatively decentralized, with several parts of the city having their own miniature "downtown areas." However, the real downtown areas are Centro, the old city center, and Zona Rosa, the new business and entertainment district.

The city is located 2,200m above mean sea level. Some people may have breathing difficulties at high altitudes and experience difficulty when breathing. The altitude is equivalent to more than 7,200 ft. This is far higher than any metropolitan area in the United States. If you live closer to sea level, you may experience difficulty breathing due to altitude and pollution. Air quality has, however, been improved in the last few years.

Also, when going out, check the date, since this is an important indicator of how full places will generally be and how long you might have to wait to get in. Salaries are usually paid twice per month: the 30th/31st-1st and the 14th-15th. On or soon after these dates is when most Mexicans will go out, especially if payday coincides with a weekend. In the more expensive places, people might leave for Acapulco or vacations farther afield during the summer and long weekends. Mexican weekends, in the sense of when it is common to go out drinking, are Thursday night to Sunday morning and sometimes throughout Sunday.