Maricopa is one of the 15 counties in the state of Arizona. It was created in 1871 from parts of Pima and Yavapai counties. As of the 2010 census, the population was 3,817,117, well up from the 2000 population of 3,072,149, and more than 10 times the 1950 population of 331,770. It is the most populous of the state's 15 counties, and contains more than 50% of the Arizona's population. Maricopa is the fourth most populous county in the US.
The county seat and largest city is Phoenix, which also doubles as the state's capital. The county has a total area of 9,224 square miles (23,890 km2), of which 9,200 square miles (24,000 km2) is land. This makes it the 15th largest county by area in the US. Maricopa is bordered on the north by Yavapai, on the east by Gila, southeast by Pinal, south by Pima and on the west by Yuma and La Paz counties.
Maricopa is served by Interstates 8,10 and 17.