Quincy, known as Illinois's "Gem City," is a city in and the county seat of Adams County, Illinois, United States, located on the Mississippi River. The 2010 census counted a population of 40,633 in the city itself, up from 40,366 in 2000. As of July 1, 2015, the Quincy Micro Area had an estimated population of 77,220.During the 19th century, Quincy was a thriving transportation center as riverboats and rail service linked the city to many destinations west and along the river. It was Illinois' second-largest city, surpassing Peoria in 1870. The city has several historic districts, including the Downtown Quincy Historic District and the South Side German Historic District, which display the architecture of Quincy's many German immigrants from the late 19th century.