The New York City Subway is a rapid transit system owned by the City of New York and leased to the New York City Transit Authority , a subsidiary agency of the Metropolitan Transportation Authority and also known as MTA New York City Transit. It is one of the most extensive public transportation systems in the world, with 468 reported passenger stations, (or 422 if stations connected by transfers are counted as one), of routes translating into 656 miles (1056 km) of revenue track, and a total of 842 miles (1355 km) including non-revenue trackage. The subway is also notable for being among the few rapid transit systems in the world to run 24 hours a day, 365 days a year. It is the only system to hold that distinction among the ten busiest systems in the world in terms of annual passenger traffic, setting it apart from cities such as London, Paris, Tokyo, Moscow, and Mexico City.

Though it is known as "the subway", implying underground operations, about 40% of the system runs on above-ground right-of-way (the system is almost entirely underground in Manhattan, as well as portions in the other boroughs), including steel or cast iron elevated structures, concrete viaducts, embankments, open cuts and surface routes. All of these construction methods are completely grade-separated from road and pedestrian crossings, and most crossings of two subway tracks are grade-separated with flying junctions.

Currently, the first phase of the Second Avenue Subway in the Upper East Side of Manhattan is being built to provide relief to the overcrowded IRT Lexington Avenue Line.