One concept which measures the world's largest cities 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.

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.

Based on their consistently defined metropolitan area criteria, they tabulate a list of the twenty largest metropolitan areas in 2003. 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.

The question of determining the world's largest cities does not allow a single, simple answer. It depends on which definitions of "city" and "size" are used, and how those definitions are applied. Complex political/cultural/social situations, sometimes controversial or disputed, further confuse the discussion. Debate on this field is highly vulnerable to bias or manipulation, as people tend to prefer whichever definition most flatters their own city.

The "size" of a city can refer to either its land area or, more typically, its population.

The boundaries of a city can also be defined in any of several ways:

The following is a list of the most populous incorporated places in the United States. As defined by the United States Census Bureau, an "incorporated place" includes a variety of designations, including a city, town, village, borough, and municipality.

Some census-designated places may also be included in the Census Bureau's listing of incorporated places. Consolidated city-counties represent a distinct type of government that includes the entire population of a county, or county equivalent. Some consolidated city-counties, however, include multiple incorporated places. This list presents only that portion (or "balance") of such consolidated city-counties that are not a part of another incorporated place.

Note that this list refers only to the population of individual municipalities within their defined limits, which does not include other municipalities or unincorporated suburban areas within urban agglomerations. A different ranking is evident when considering U.S. metropolitan area populations.

The following table lists the incorporated places in the United States with a population on July 1, 2009, of more than 100,000 as estimated by the United States Census Bureau. Population densities are also given by the U.S. Census Bureau, but are listed as of the 2000 U.S. Census.

Since the U.S. Census Bureau typically does not rank the cities of U.S. territories along with those of the fifty states and the District of Columbia, the cities of Puerto Rico with populations greater than 100,000 are listed separately below.

A city is a relatively large and permanent settlement. Although there is no agreement on how a city is distinguished from a town within general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.

For example, an article of incorporation approved by the local state legislature distinguishes a city government from a town in Massachusetts. In the United Kingdom and parts of the Commonwealth of Nations, a city is traditionally a settlement with a royal charter. Historically, in Europe, a city was understood to be an urban settlement with a cathedral.

Cities generally have advanced systems for sanitation, utilities, land usage, housing, and transportation. The concentration of development greatly facilitates interaction between people and businesses, benefiting both parties in the process. A big city or metropolis usually has associated suburbs and exurbs. Such cities are usually associated with metropolitan areas and urban sprawl, creating numerous business commuters traveling to urban centers of employment. Once a city sprawls far enough to reach another city, this region can be deemed a conurbation or megalopolis.