Nassau County is a suburban county on Long Island in the U.S. state of New York. At the 2010 census, the county's population was 1,339,532, estimated to have increased to 1,361,350 in 2015. The county seat is located in the Village of Garden City, within the boundaries of the Mineola 11501 zip code.

Located immediately east of New York City, within the New York-Newark-Bridgeport, the county is one of the four counties that occupy Long Island, together with Suffolk County to its immediate east and Queens and Kings counties to the west, which correspond, respectively, to the New York City boroughs of Queens and Brooklyn. Two cities, three towns, 64 incorporated villages, and more than 60 unincorporated hamlets are located within the county. There are 56 public school districts within the county. Post office districts and school districts use the same names as a city, hamlet, or village within them, but each sets the boundaries independently.

In 2012, Forbes magazine, in an article based on the U.S. Census Bureau's American Community Survey, reported that Nassau County was one of the highest income counties in the United States and the most affluent in the state of New York, comprising four of the nation's top ten towns by median income. It also ranks as the most expensive county in America. Nassau County also has a designated police department, fire commission, and elected executive and legislative bodies.