"I Know What You Did Last Summer" is a song by Canadian singer Shawn Mendes and Cuban-American singer Camila Cabello. It was released as the lead single from his 2015 revisited version of his debut studio album, Handwritten (2015) via Island Records on November 18, 2015. It reached the top 20 on the US Billboard Hot 100 and on the Canadian Hot 100. Mendes and Cabello promoted the song with several televised performances, including on The Tonight Show Starring Jimmy Fallon and at the 2016 People's Choice Awards.

"I Know What You Did Last Summer" was written backstage spontaneously on one of Taylor Swift's The 1989 World Tour stops, in which Mendes was an opening act and Cabello was the surprise guest as part of the group Fifth Harmony. "We were just backstage. There were like 100 people in my dressing room at the time, and it was madness back there, but I had this guitar and I was just jamming out, and we basically wrote a pre-chorus and a chorus in like 30 minutes," Mendes said in a video released about the making of this song. "And we planned to go into the studio, and it was like an 11-hour studio session where we finished writing the song." After a live performance on Live with Kelly and Michael, Cabello said, "We were just kind of jamming out. We weren’t really consciously writing a song. We kind of started spitting out melodies and put lyrics to it; I don't think we knew we had a song."

Cabello described "I Know What You Did Last Summer" as a "conversation between two people in a relationship where it's dying, but nobody wants to admit that it's dying."

"I Know What You Did Last Summer" contains elements from Bill Withers' 1971 single "Ain't No Sunshine", thus songwriting credits was given to Withers. The song is sung in A minor, with a tempo of 114 bpm and a time signature of 4
4.

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