San Antonio is the second-most populous city in the state of Texas after Houston and seventh-most populated in the United States. As of the 2005 U.S. Census estimate, the city had a population of over 1.2 million. Its eight-county metropolitan area has a population of over 1.8 million, the twenty-ninth most-populated metro area in the United States. San Antonio, which is the county seat of Bexar County, covers over 400 square miles on the northern edge of the South Texas region and southeast of the Texas Hill Country.

According to the United States Census Bureau, in 2000 the city had a total area of 412.07 square miles (1,067.3 km²) — 407.56 square miles (1,055.6 km²) of it is land and 4.51 square miles (11.7 km²) of it is water. The city sits on the Balcones Escarpment. San Antonio was named for the Portuguese Saint Anthony of Padua, whose feast day it was when a Spanish expedition stopped in the area in 1691. The city has a strong military presence—it is home to Fort Sam Houston, Lackland Air Force Base, Randolph Air Force Base, and Brooks City-Base. San Antonio is home to the South Texas Medical Center, the largest and only medical research and care provider in the South Texas region.