Houston is the largest city in the state of Texas and the fourth-largest in the United States. The city covers more than 600 square miles (1,600 km²) and is the county seat of Harris County. As of the 2005 U.S. Census estimate, Houston had a population of more than 2 million. The city is at the heart of the Houston–Sugar Land–Baytown metropolitan area, the largest cultural and economic center of the Gulf Coast region and the seventh-largest metropolitan area in the U.S. with a population of 5.3 million in 10 counties.
Houston is internationally known for its energy—particularly oil—and aeronautics industries, and for its ship channel. The Port of Houston ranks first in the country in international commerce and is the sixth-largest port in the world. Second only to New York City in Fortune 500 headquarters, Houston is the seat of the Texas Medical Center, which contains the world's largest concentration of research and healthcare institutions.