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), 229 miles (369 km) 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. The system itself has more annual boardings than those in cities such as London, Paris, and Mexico City. It has nearly twice as many daily riders as every other rapid transit system in the United States combined.

Though it is known as "the subway", implying underground operations, about 40% of the system runs on above-ground tracks (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.

Subway stations are located throughout Manhattan, Brooklyn, Queens, and the Bronx. All services pass through Manhattan, except for the Brooklyn–Queens Crosstown Local (G), which connects Brooklyn and Queens directly without entering Manhattan; the Franklin Avenue Shuttle; and the Rockaway Park Shuttle. All but two of the 468 stations of the subway are served 24 hours a day. This is very rare globally and only found in the United States with PATH (connecting New Jersey with Manhattan), PATCO (linking Philadelphia with southern New Jersey), and two lines of the Chicago 'L'.

In 2005, the New York City Subway hit a 50-year record in usage, with ridership of 1.45 billion.In the fourth quarter of 2007, average weekday ridership was 6,432,700 making it the highest figure since such numbers were first recorded in 1970.The trend toward higher ridership has continued into 2008; MTA has released figures that subway use was up 6.8 percent for January and February as energy prices have encouraged riders to use mass transit over automobiles.

According to the United States Department of Energy, energy expenditure on the New York City Subway rail service was 3492 BTU/passenger mile (2289 kJ/passenger km) in 1995. This compares to 3702 BTU/passenger mile (2427 kJ/passenger km) for automobile travel.One should note as well that the figure for automobiles is averaged over the entire United States. Driving a car in New York City is probably on the whole significantly less efficient, because of the high concentration of traffic lights and vehicular traffic.

Many lines and stations have both express and local service. These lines have three or four tracks: normally, the outer two are used for local trains, and the inner one or two are used for express trains. Stations served by express trains are typically major transfer points or destinations. The BMT Jamaica Line uses skip-stop service on portions, in which two services operate over the line during rush hours, and minor stations are only served by one of the two. The IRT Broadway–Seventh Avenue Line used skip-stop until May 27, 2005. On that date, the IRT train was eliminated.

An underground transit system in New York City was first built by Alfred Ely Beach in 1869. His Beach Pneumatic Transit only extended 312 feet (95 m) under Broadway and exhibited his idea for a subway. The tunnel was never extended, although extensions had been planned to take the tunnel southward to The Battery and northwards towards the Harlem River. It was demolished when the BMT Broadway Line was built in the 1910s.

The first underground line of the subway opened on October 27, 1904, almost 35 years after the opening of the first elevated line in New York City, which became the IRT Ninth Avenue Line. The oldest structure still in use today opened in 1885 as part of the Lexington Avenue Line, and is now part of the BMT Jamaica Line in Brooklyn. The oldest right-of-way, that of the BMT West End Line, was in use in 1863 as a steam railroad called the Brooklyn, Bath and Coney Island Rail Road. The Staten Island Railway, which opened in 1860, currently utilizes R44 subway cars, but it has no links to the rest of the system and is not usually considered part of the subway proper.

By the time the first subway closed, the lines had been consolidated into two privately owned systems, Brooklyn Rapid Transit Company (BRT, later Brooklyn-Manhattan Transit Corporation, BMT) and Interborough Rapid Transit Company (IRT). The city was closely involved: all lines built for the IRT and most other lines built or improved for the BRT after 1913 were built by the city and leased to the companies. The first line of the city-owned and operated Independent Subway System (IND) opened in 1932; this system was intended to compete with the private systems and allow some of the elevated railways to be torn down, but was kept within the core of the City due to the low amount of startup capital provided to the Board Of Transportation by the state. This required it to be run 'at cost', necessitating fares up to double the five cent fare popular at the time.

In 1940, the two private systems were bought by the city; some elevated lines closed immediately, and others closed soon after. Integration was slow, but several connections were built between the IND and BMT, and they now operate as one division called the B Division. Since the IRT tunnel segments and stations are too narrow to accommodate B Division cars, as well as curves too sharp for B Division cars, the IRT remains its own division, A Division. C Division consists of non-revenue maintenance cars, built to IRT specs in order to provide maintenance to all of the subway system.

The New York City Transit Authority was created in 1953 to take over subway, bus, and streetcar operations from the city, and was placed under control of the Metropolitan Transportation Authority in 1968. In 1934, the BRT, IRT, and IND transit workers unionized into Local 100 of the Transport Workers Union. Since then, there have been three union strikes. In 1966, transit workers went on strike for 12 days, and again in 1980 for 11 days. On December 20, 2005, transit workers again went on strike over disputes with MTA regarding salary, pensions, retirement age, and health insurance costs. That strike lasted just under three days.

When the IRT subway debuted in 1904, typical tunnel construction was the cut-and cover method. The street was torn up to dig the tunnel below, then the street was rebuilt above. This method worked well for digging soft dirt and gravel near the street surface. However, tunnel boring machines were required for thicker sections made of bedrock, such as the Harlem and East River tunnels, which used cast-iron tube, and the segments between 33rd and 42nd streets under Park Avenue, between 116th Street and 120th Street under Broadway, and between 157th Street and Fort George under Broadway and Eleventh Avenue, all of which used either rock or concrete-lined tunnels.

Many rapid transit systems run relatively static routings, so that a train "line" is more or less synonymous with a train "route". In New York, routings change often as new connections are opened or service patterns change. The "line" describes the physical railroad line or series of lines that a train "route" uses on its way from one terminal to another.

"Routes" (also called "services") are distinguished by a letter or a number. "Lines" have names.

There are 26 train services in the subway system, including three short shuttles. Each route has a color, representing the Manhattan trunk line of the particular service and is labeled as local or express. A different color is assigned to the Crosstown Line route, since it does not operate in Manhattan, and shuttles are all colored dark gray. Each service is also named after its Manhattan (or crosstown) trunk line.

Though all but two subway stations are served on a 24-hour basis, some of the designated routes do not run during the late night hours or use a different routing during those hours. In addition to these regularly scheduled changes, because there is no nightly shutdown for maintenance, tracks and stations must be maintained while the system is operating. In order to accommodate such work, services are sometimes re-routed during the overnight hours or on weekends.

The current color system depicted on official subway maps was proposed by R. Raleigh D'Adamo, a lawyer who entered a contest sponsored by the Transit Authority in 1964. D'Adamo proposed replacing a map that used only three colors (representing the three operating entities of the subway network) with a map that used a different color for each line. D'Adamo's contest entry shared first place with two others and led the Transit Authority to adopt a multi-colored scheme. (D'Adamo subsequently earned a master's degree in transportation planning and engineering from Polytechnic University and worked for transit authorities, including a stint at the MTA, and was responsible for organizing and building what today is the Westchester County Bee-Line bus system.)[18] However, the lines are not referred to by color (e.g., Blue line or Green line), although the colors are often assigned through their groups ( and are blue whereas the and are green).

A typical subway station has waiting platforms ranging from 500 to 600 feet (150 to 180 m) long to accommodate large numbers of people. Passengers enter a subway station through stairs towards station booths and vending machines to buy their fare, which is currently stored in a MetroCard. After swiping the card at a turnstile, customers continue to the platforms. Some subway lines in the outer boroughs and northern Manhattan have elevated tracks with stations to which passengers climb up via stairs, escalator, or elevator.

At the top of most of the system's subway stations sits a lamp post or two bearing a colored spherical lamp. Before the introduction of the MetroCard in 1994, these lights indicated the station's availability. A green lamp meant that the station was open and running 24 hours a day, a yellow lamp meant that it was open only during the day, while a red lamp meant that it was an exit only. The yellow lamp was eventually phased out, being replaced by red lamps. Today, this color system uses green lamps to indicate 24 hour entrances and red lamps to indicate non 24-hour entrances.

Due to the large number of transit lines, one platform or set of platforms often serves more than one service (unlike other rapid transit systems, including the Paris Metro but like some lines on the London Underground). A passenger needs to look at the signs hung at the platform entrance steps and over each track to see which trains stop there and when, and at the arriving train to see which train it is.

There are a number of platform configurations possible. On a 2-track line, a station may have one center platform used for trains in both directions, or 2 side platforms, one for a train each direction. For a 3-track or 4-track line, local stops will have side platforms and the middle one or two tracks will not stop at the station. For most 3- or 4-track express stops, there will be two island platforms, one for the local and express in one direction, and another for the local and express in the other direction. In a 3-track configuration, the center track can be used toward the center of the city in the morning and away from the center in the evening, though not every 3-track line has that express service.

In a few cases, a 4-track station has an island platform for the center express tracks and two side platforms for the outside local tracks. This occurs only at three stations near major railway stations where the next station along the line is also an express station with the more common platform configuration. The purpose of splitting the platforms is to prevent through riders from adding to the station's crowding by transferring from local to express or from express to local. This occurs at Atlantic Avenue on the 2/3/4/5 Lines with adjacent express station Nevins Street, and 34th St.-Penn Station on both the 1/2/3 Lines and A/C/E Lines, with adjacent express stations at 42nd Street. This does not occur at Grand Central on the 4/5/6 Lines, which has no adjacent express station. Almost everywhere expresses run, they run on the inner one (of 3) or two (of 4) tracks, and locals run on the outer two tracks. There is one notable 6-track station, DeKalb Avenue, where trains to or from the Manhattan Bridge either stop at the outer tracks of one of the island platforms ("local tracks"), or pass through the station on the middle tracks ("super express tracks"). Trains to or from the Montague Street Tunnel stop across the platform from the respective outer track ("express tracks").

Many stations are decorated with intricate ceramic tile work, some of it dating back to 1904 when the subway first opened for business. The subway tile artwork tradition continues today. The Arts for Transit program oversees art in the subway system. Permanent installations, such as sculpture, mosaics, and murals; photographs displayed in lightboxes, and musicians performing in stations encourage people to use mass transit. In addition, commissioned posters are displayed in stations and "art cards", some displaying poetry, are in many of the trains themselves in unused advertisement fixture slots. Some of the art is by internationally-known artists such as David Hockney.

Most stations are not handicapped accessible. The exceptions are newly constructed or extensively renovated stations called "key stations", as required by the ADA. See New York City Subway accessibility for more details.