In the United States, the Office of Management and Budget (OMB) has produced a formal definition of metropolitan areas. These are referred to as "Metropolitan Statistical Areas" (MSAs) and "Combined Statistical Areas." An earlier version of the MSA was the "Standard Metropolitan Statistical Area" (SMSA). MSAs are composed of counties and for some county equivalents . In New England, because of the greater importance of towns over counties, similar areas are defined based on town units, known as New England City and Town Areas (NECTAs).

MSAs are delineated on the basis of a central urbanized area—a contiguous area of relatively high population density. The counties containing the core urbanized area are known as the central counties of the MSA. Additional surrounding counties (known as outlying counties) can be included in the MSA if these counties have strong social and economic ties to the central counties as measured by commuting and employment. Note that some areas within these outlying counties may actually be rural in nature.

MSAs are used for official purposes, but they are not the only estimates of metro area populations available. The appropriate figures for some metro areas are much debated, and in some cases reputable sources provide figures which differ by millions. The most contentious examples include the Greater Los Angeles Area, the San Francisco Bay Area, and Greater Cleveland. The official definitions used for the last U.S. Census differed from those for previous censuses, making comparisons difficult even between official figures at different dates (comparing 2000 with 1990, Baltimore was separated from Washington, D.C., but West Palm Beach was combined with Miami-Fort Lauderdale, which made a considerable difference to the rankings of both metros). Care should also be taken when comparing MSA figures with population figures for cities or metro areas outside the U.S., which may be based on substantially different boundary systems and definitions of terms. Additionally, MSA boundaries do not stretch into neighboring Canada or Mexico, so the actual metropolitan populations of border cities such as Detroit, Buffalo, El Paso and San Diego are often substantially larger than their MSA figures.

As of June 2003, there is now an additional classification, that of a “Metropolitan Division.” The term metropolitan division is used to refer to a county or group of closely-tied contiguous counties that serve as a distinct employment region within a metropolitan statistical area that has a population core of at least 2.5 million. While a metropolitan division is a subdivision of a larger metropolitan statistical area, it often functions as a distinct social, economic, and cultural area within the larger region.

The following is a list of the 25 most populated metropolitan statistical areas and metropolitan divisions in the United States, according to the July 1, 2006 U.S. Census Bureau estimates:general/2006/CBSA-EST2006-01.csv" target="_blank">http://www.census.gov/population/www/estimates/metro_general/2006/CBSA-EST2006-01.csv