Westchester County is a county in the U.S. state of New York. It is the second-most populous county on the mainland of New York, after the Bronx, and the most populous county in the state north of New York City. According to the 2010 Census, the county had a population of 949,113, estimated to have increased by 3.3% to 980,244 by 2017. Situated in the Hudson Valley, Westchester covers an area of 450 square miles (1,200 km2), consisting of six cities, 19 towns, and 23 villages.
Westchester became the first suburban area of its scale in the world to develop, due mostly to the upper-middle class development of entire communities in the late 19th century, and the subsequent rapid population growth.