An Edge City is an American term for a concentration of business, shopping, and entertainment outside a traditional downtown (or central business district) in what had previously been a residential or rural area. The term was popularized in the 1991 book Edge City: Life on the New Frontier by Joel Garreau, who established its current meaning while working as a reporter for the Washington Post. Garreau argues that the edge city has become the standard form of urban growth worldwide, representing a 20th-century urban form unlike that of the 19th-century central downtown. Other terms for the areas include suburban activity centers, megacenters, and suburban business districts. 
Garreau established five rules for a place to be considered an edge city:
- Has five million or more square feet (465,000 m²) of leasable office space.
- Has 600,000 square feet (56,000 m²) or more of leasable retail space.
- Has more jobs than bedrooms.
- Is perceived by the population as one place.
- Was nothing like a "city" as recently as 30 years ago. Then it was just bedrooms, if not cow pastures."
Most edge cities develop at or near existing or planned freeway intersections, and are especially likely to develop near major airports. They rarely include heavy industry. They often are not separate legal entities but are governed as part of surrounding counties (this is more often the case in the East than in the Midwest, South, or West). They are numerous—almost 200 in the United States, compared to 45 downtowns of comparable size—and are large geographically because they are built at automobile scale.
Spatially, edge cities primarily consist of mid-rise office towers (with some skyscrapers) surrounded by massive surface parking lots and meticulously manicured lawns, almost reminiscent of the designs of Le Corbusier. Instead of a traditional street grid, their street networks are hierarchical, consisting of winding parkways (often lacking sidewalks) that feed into arterial roads or freeway ramps. However, edge cities feature job density similar to that of secondary downtowns found in places such as Newark and Pasadena; indeed, Garreau writes that edge cities' development proves that "density is back".
Garreau identified three distinct varieties of the edge city phenomenon:
- Boomers – The most common type, having developed incrementally around a shopping mall or highway interchange. 
- Greenfields – Having been master-planned as new towns, generally on the suburban fringe. 
- Uptowns – Historic activity centers built over an older city or town (sometimes a satellite city). 
The edge city is fundamentally impossible without the automobile. It was not until automobile ownership surged in the 1950s, after four decades of fast steady growth, that the edge city became truly possible. Whereas virtually every American central business district (CBD) or secondary downtown that developed around non-motorized transportation or the streetcar has a pedestrian-friendly grid pattern of relatively narrow streets, most edge cities instead have a hierarchical street arrangement centered around pedestrian-hostile arterial roads.
Perhaps the first edge city was Detroit's New Center, developed in the 1920s. Located three miles (5 km) north of the city's downtown (but within Detroit's city limits), this was developed as an attempt to relocate downtown Detroit. New Center and the Miracle Mile section of Wilshire Boulevard in Los Angeles are considered the earliest automobile-oriented urban forms. However the two were built with radically different purposes in mind (New Center as an office park, the Miracle Mile as a retail strip). Garreau's classic example of an edge city is the information technology center, Tysons Corner, Virginia, west of Washington, DC. As recently as the end of World War II, it was a country crossroads, but it now has more office space than downtown Atlanta.
Edge cities planned around freeway interchanges have a history of suffering severe traffic problems if one of these freeways goes unbuilt. In particular, Century City, a pioneering edge city built on former 20th Century Fox backlot in western Los Angeles, was built with the long-term intent of establishing connections to both a citywide light rail or monorail system and the planned Beverly Hills Freeway. Neither project ever came to fruition, resulting in massive congestion on the surface streets connecting Century City to the San Diego (I-405) and Santa Monica (I-10) freeways, each two miles (3 km) distant. Calls by former Los Angeles mayor Antonio Villaraigosa for construction of a Wilshire Boulevard extension of the Purple Line subway led many transportation planners and Century City occupants and neighbors to call for a southerly routing of the extension that would pass by Century City on its northern leg.
Edge cities may turn out to have been only a 20th-century phenomenon because of their limitations.  The residents of the low-density housing areas around them tend to be fiercely resistant to their outward expansion (as has been the case in Tyson's Corner and Century City), but because their internal road networks are severely limited in capacity, densification is far more difficult than in the traditional grid network that characterizes traditional CBDs and secondary downtowns. As a result, construction of medium- and high-density housing in edge cities ranges from difficult to impossible. Because most are built at automobile scale, mass transit frequently cannot serve them well. Pedestrian access to and circulation within an edge city is impractical if not impossible, even if residences are nearby. Revitalization of edge cities may be "the major urban renewal project of the 21st century". An example of this can be seen in Western Europe, particularly in France, Spain and Germany where, in a reversal of the situation in most US cities, the downtown is upscale and some suburbs (banlieues for example) are the slums.
Despite the lessons of the American experience, in rapidly developing countries such as China and India and the United Arab Emirates, the edge city is quickly emerging as an important new development form as automobile ownership skyrockets and marginal land is bulldozed for development. For example, the outskirts of Bangalore, India are increasingly replete with mid-rise mirrored-glass office towers set amid lush gardens and sprawling parking lots where many foreign companies have set up shop. Dubai offers another example.
- Dunphy, Robert T (1999). "Activity Centers". In Edwards, John D. Transportation Planning Handbook (2nd ed. ed.). Washington, DC: Institute of Transportation Engineers. ISBN 9780935403336.
- Garreau, Joel (1992). Edge city : Life on the New Frontier (1st Anchor Books ed. ed.). New York: Anchor Books. ISBN 978-0385424349.
- Lang, Robert E.; LeFurgy, Jennifer. "Edgeless cities: Examining the Noncentered metropolis". Housing Policy Debate 14 (3): 427–460. doi:10.1080/10511482.2003.9521482.
- Garreau's web site with searchable text of the book
- John McCrory, The Edge City Fallacy: New Urban Form or Same Old Megalopolis?.
- Joel Garreau's vision of the future of edge cities: Edgier Cities