WLIW (channel 21) is a secondary PBS member television station licensed to Garden City, New York, United States, serving the New York City television market. It is owned by The WNET Group alongside the area's primary PBS member, Newark, New Jersey–licensed WNET (channel 13); two Class A stations, WNDT-CD (channel 14) and WMBQ-CD (channel 46, which shares spectrum with WLIW); and NPR member WLIW-FM (88.3) in Southampton.

WLIW and WNET share studios at One Worldwide Plaza in Midtown Manhattan with an auxiliary street-level studio in the Lincoln Center complex on Manhattan's Upper West Side. WLIW's transmitter is located at One World Trade Center; the station also maintains a production studio at its former transmitter site in Plainview, New York. WLIW's multiplex is New York's high-power ATSC 3.0 (NextGen TV) television station and also broadcasts WMBQ-CD.

WLIW was established in 1969 as the first television station on Long Island. Originally operated on a tight budget, the station had no permanent studio facilities for nearly a decade. In the 1980s and 1990s, increasing cable television coverage led to the expansion of WLIW into a regional service that was the smaller competitor to WNET, the nation's largest public TV station, and the station increased its own programming efforts. However, some critics felt that this shift deemphasized the station's Long Island identity. In 2003, WLIW and WNET merged, completing an 18-month process. As part of the WNET Group, WLIW maintains a separate vice president and general manager, Diane Masciale, who is in charge of the entire group's locally oriented television production.

The Nassau County Board of Supervisors voted on February 14, 1968, to provide funding to set up an educational television station on Long Island, thereby also accessing matching funds from the New York state government. The Long Island Educational Television Council then applied for and, in June, received a construction permit for channel 21 at Garden City. Facilities were established on the campus of Nassau Community College, while a 60-hour broadcast week evenly split between in-school instructional and general cultural offerings was slated. Test programming from Long Island's first TV station was aired beginning on January 14, 1969, with the station still not completely set up and technicians using screwdrivers to adjust audio levels including a series of hearings on the Long Island Rail Road. Official broadcasting did not begin until January 27.

WLIW operated on a very tight budget—so tight that its founding general manager, public television veteran William Pearce, resigned after four months to return to his prior employer, WXXI-TV in Rochester. It lacked a full studio of its own or its own mobile broadcasting equipment, and it spent seven months without a new leader as other public television managers turned down the post. Color telecasts only began with the installation of color video tape machines in December 1972, nearly four years after the station started up. The station finally got studio space when it moved in to the New York Institute of Technology (NYIT) in Westbury in 1974, but that arrangement lasted two years. Station operations were then moved into a mobile van, which some employees claimed was due to a vote by technicians to unionize. As the station investigated studio space at Stony Brook University, it also received federal approval and matching grants to move its transmitter to Plainview and increase power to cover all of Long Island.

A year of major change would mark 1979 for WLIW. In February, the new Plainview transmitter site and studios, at the highest point on Long Island, was activated, significantly improving reception and extending channel 21's reach and capabilities. However, internal strife dominated the second half of the year. Charles R. Bell, who had been general manager since Pearce's departure, accused some of the station's trustees with interfering in programming decisions to further political ambitions and the goals of a political strategy firm which one of them headed. The trustees responded by voting not to renew Bell's contract.

John Wicklein assumed the manager post in February 1980 and sought to give the station an identity independent from that of WNET and additional local programming and support. He was also tasked with erasing a $250,000 deficit from the building programs of the late 1970s that had forced layoffs and program suspensions. When other public television stations in the state visited WLIW, their leaders assessed additional non-financial problems at the station: an acceptance of living in the shadow of WNET and a "defeatist" attitude. Its ability to attract local support was eclipsed by stations serving far fewer people, such as WCFE-TV in Plattsburgh. Wicklein left after three months and soon was replaced by Arthur Gillick of Syracuse. Gillick was able to steady the station's financial picture and restore the lost local programs as a result, though further local cuts led to the loss of the station's news programming for a time beginning in 1982. He served as general manager until his December 1983 death from cancer at the age of 35.

The 1980s would bring expanded coverage for WLIW in New York, New Jersey, and Connecticut, thanks to increased carriage on cable systems. A station that in 1985 was still trying to recover from its attempt to be a "junior Channel 13" in the eyes of William Renn, a professor at Hofstra University, had by 1990 leaned into its growing reach, with 62 percent of its members coming from outside Long Island and a viewership that put it in the top ten nationally among public TV stations. It also was profitable for the first time in its history. Despite this, political leaders on Long Island continued to clamor for increased local programming. The station also had to fight for its expanded cable carriage after must-carry rules were abolished in 1985. Some cable providers dropped the station citing duplication to WNET and to the station's over-the-air broadcast. Paragon Cable's Manhattan system dropped WLIW in 1987 and replaced it with the Cable Value Network, a home shopping channel, only to restore it weeks later after protests from subscribers.