"Knock Knock" is a song by American recording artist Monica, taken from her fourth studio album After the Storm (2003). It was written and produced by rapper-producer Missy Elliott and commissioned following the delay and subsequent reconstruction of Monica's 2002 album, All Eyez on Me. One out of a handful of tracks which Elliott contributed, the song incorporates elements of 1970s-style soul and quiet storm, as well as hip hop, and is built around excerpts of the record "It's a Terrible Thing to Waste Your Love" as written by Lee Hatim and released by The Masqueraders in 1976. Rapper Kanye West, whose 2005 Freshmen Adjustment mixtape recording "Apologize" the track is based on, is listed as the song's co-producer.