Regional definitions vary from source to source. The states shown in dark red are usually included, while all or portions of the striped states may or may not be considered part of the Southwestern United States.

The Southwestern United States (called the American Southwest or simply the Southwest) is a portion of the country mostly considered located in the western region of the United States. It is warmer than the northern states and drier than the eastern states. Depending on the source, the area almost always includes Arizona and New Mexico, All or parts of Southern California, Nevada, Utah, and Colorado may also be trans-pecos "horn" of far western Texas. The nearby striped states may also be conidered part of this region, as may the trans-pecos "horn" portion of far western Texas. The striped states of California,Nevada, Utah, and Colorado are all also classified as part of the "West" by the United States Census Bureau, while Texas and Oklahoma are considered part of the "South" by the same Bureau(see "United States Census Bureau Regions and Divisions"). The Southwest is growing. Big cities in the area are Phoenix, Las Vegas, El Paso, Albuquerque, and Tucson. It is one of the fastest growing parts of the country.