Knock Knock is the seventh album by Bill Callahan (also known as Smog), released in January 1999 on Drag City and by Domino in Europe. The album is the fourth and final collaboration with the producer and musician Jim O'Rourke. Knock Knock elaborates Callahan's sound and provides twists and edges to the folky style of its predecessor. "Held" was the first single, followed by "Cold Blooded Old Times", which later appeared on the High Fidelity film soundtrack, thereby attracting some attention to Callahan's work. Other soundtrack appearances include "Teenage Spaceship" in Crazy (2000, directed by Hans-Christian Schmid) and "Hit the Ground Running" in Swimming (2000, directed by Robert J. Siegel). The album was accompanied by the release of the single "Look Now", with the two Japanese bonus tracks.
Loren Mazzacane Connors plays guitar on "Teenage Spaceship" and"Sweet Treat", Thymme Jones plays drums, Jim O'Rourke plays piano and "Wurley" plays guitar.
Callahan has said the cover is a reference to "two zen tenets - that of life being like a flash of lightning or a wildcat's roar" (Smog interview in Ink Blot magazine), though others have speculated that it is a reference to Cat Power, who had been in a relationship with Callahan immediately prior to the writing of the album.