The IND Eighth Avenue Line is a rapid transit line in the B Division of the New York City Subway. Opened in 1932, it was the first line of the Independent Subway System (IND); as such, New Yorkers originally applied the Eighth Avenue Subway name to the entire IND system.

The line runs from 207th Street in Inwood south to an interlocking south of High Street in Brooklyn Heights, including large sections under St. Nicholas Avenue, Central Park West, and Eighth Avenue. The entire length is underground, though the 207th Street Yard, which branches off near the north end, is on the surface. Flying junctions are provided with the IND Concourse Line, IND Sixth Avenue Line, and IND Queens Boulevard Line.

Most of the line has four tracks, with one local and one express track in each direction, except for the extreme north and south ends, where only the two express tracks continue. Internally, the line is chained as Line "A", with tracks A1, A3, A4, and A2 from west to east, running from approximately 800 at the south end to 1540 at the north end (measured in hectofeet).

The whole line is served at all times by the A train, which runs express except during late nights. The C provides local service south of 168th Street while the A runs express. In addition, the B provides weekday local service and the D full-time express service between the Concourse Line (145th Street) and Sixth Avenue Line (59th Street–Columbus Circle) junctions, and the E runs local from the Queens Boulevard Line junction at 50th Street south to World Trade Center. The A, C, and E are colored blue on signs because they run via Eighth Avenue through Midtown Manhattan, while the B and D are orange since they use the Sixth Avenue Line through Midtown Manhattan.

The following services use part or all of the Eighth Avenue Line. The trunk line's bullets are colored blue:

The Eighth Avenue Line begins as a two-track subway under Broadway at 207th Street in Inwood. A flying junction just to the south brings two tracks from the 207th Street Yard between the main tracks, merging after Dyckman Street. The subway leaves Broadway to pass under Fort Tryon Park to the north end of Fort Washington Avenue, which it follows to roughly 175th Street before turning southeast under private property.

The small 174th Street Yard lies under Broadway, with two tracks exiting to the south under that roadway. When the George Washington Bridge was designed in the 1920s, provisions were made for a lower deck that would carry these two tracks north from the yard and across the bridge, as well as two commuter rail tracks. However, when the lower level was added in 1962, it instead carried a roadway.

The two main tracks from Fort Washington Avenue enter Broadway near 172nd Street, curving and running underneath a public school (PS173M) at 174th Street, and other private property, and the yard tracks in a double-decker tunnel. A few blocks later, the lower tracks separate to straddle the yard tracks at 168th Street. The local/express split begins here, with the local tracks coming from the yard and the express tracks coming from Inwood. Contrary to standard practice, the two local tracks are in the center and the two express tracks are on the outside. Except during late nights, the local service (C) ends at 168th Street, reversing direction on the yard tracks; the A runs to 207th Street at all times, express except during late nights. South of 168th Street, the express (outer) tracks lower below the local tracks, forming another double-decker tunnel, this time under St. Nicholas Avenue.