I Know What You Did Last Summer

I Know What You Did Last Summer is a 1997 American slasher film directed by Jim Gillespie, written by Kevin Williamson, and starring Jennifer Love Hewitt, Sarah Michelle Gellar, Ryan Phillippe and Freddie Prinze Jr. It is loosely based on the 1973 novel of the same name by Lois Duncan and is the first installment in the I Know What You Did Last Summer franchise. The film centers on four young friends who are stalked by a hook-wielding killer one year after covering up a car accident in which they supposedly killed a man. The film also draws inspiration from the urban legend known as "The Hook" and the 1980s slasher films Prom Night (1980) and The House on Sorority Row (1982).

Prior to Scream (1996), Williamson was approached to adapt Duncan's source novel by producer Erik Feig. Where Williamson's screenplay for Scream contained prominent elements of satire and self-referentiality, his adaptation of I Know What You Did Last Summer reworked the novel's central plot to resemble a straightforward 1980s-era slasher film.

I Know What You Did Last Summer was released theatrically on October 17, 1997. It received mixed reviews from critics, but was commercially successful, grossing $125.3 million worldwide on a budget of $17 million, and remaining number 1 at the U.S. box office for three consecutive weeks. It was also nominated for and won multiple awards. The film has also been parodied and referenced in popular culture, and is credited alongside Scream with revitalizing the slasher genre in the 1990s.

The film was followed by a direct sequel, I Still Know What You Did Last Summer (1998), with the main surviving characters returning. Later, a direct-to-video sequel was made called I'll Always Know What You Did Last Summer (2006), without featuring any of the original cast. At one point, a remake was planned. In 2021, a television series was released. As of 2023, a sequel is in the works.