Houston is the largest city in Texas and the fourth-largest city in the United States. It is the county seat of Harris County and the economic center of the Houston–The Woodlands–Sugar Land metropolitan area, which is the fifth-most populous metropolitan area in the United States. Houston is known for its energy industry—particularly for oil and natural gas—as well as for biomedical research and aeronautics. The city has a population of over 2 million people and is the most populous city in Texas.