The Inland Empire is mainly located in Southeast California, particularly the Riverside and San Bernardino counties. It is the second largest metropolitan area in Southern California, third in California, 14th largest in the United States and 25th in the Americas. It generally encompasses the urbanized, western areas of those counties. The Inland Empire is centered in the region's oldest cities: Ontario, San Bernardino, and Riverside.
The "Inland" part of the name is derived from the region's location about 37 miles (60 km) inland from the Pacific Ocean (from Huntington Beach) and east of downtown Los Angeles.
Air pollution, or suspended particulate matter locally generated from the increased number of automobiles in the area, from point sources such as factories, dust carried into the air by construction activity, and the contribution of similar pollutants from the Los Angeles area has regularly caused the Inland Empire to be at, or near, the bottom of many air quality ratings. In 2004, the EPA rated the San Bernardino-Riverside area as having the worst particulate air pollution in the United States (although the San Joaquin Valley in central California had the worst overall air pollution).