The WB aired the first five seasons of Buffy the Vampire Slayer.
When Buffy premiered in 1997, it was part of The WB’s early programming identity and quickly became one of the young network’s signature shows. The network broadcast seasons one through five, covering Buffy Summers’s arrival in Sunnydale, the formation of the Scooby Gang, and major storylines involving Angel, the Mayor, and Glory.
The series did not remain on The WB for its entire run. In 2001, Buffy moved to UPN, which aired the sixth and seventh seasons. That change is the main reason viewers sometimes incorrectly choose UPN for this question: UPN was indeed the home of the show’s final two seasons, but not its first five.
The WB and UPN were separate American broadcast networks, even though both targeted younger audiences and carried many genre and teen dramas. Buffy’s network move did not reset the story or its season numbering; season six followed directly from the fifth-season finale, "The Gift," and the show concluded on UPN with "Chosen" in 2003.