The Hangover is a 2009 American comedy film directed by Todd Phillips, and written by Jon Lucas and Scott Moore. It is the first installment in The Hangover trilogy. The film stars Bradley Cooper, Ed Helms, Zach Galifianakis, Heather Graham, Justin Bartha, and Jeffrey Tambor. It tells the story of Phil Wenneck (Cooper), Stu Price (Helms), Alan Garner (Galifianakis), and Doug Billings (Bartha), who travel to Las Vegas for a bachelor party to celebrate Doug's impending marriage. However, Phil, Stu, and Alan wake up with Doug missing and no memory of the previous night's events, and must find Doug before the wedding can take place.

Lucas and Moore wrote the script after executive producer Chris Bender's friend disappeared and had a large bill after being sent to a strip club. After Lucas and Moore sold it to the studio for $2 million, Phillips and Jeremy Garelick rewrote the script to include a tiger as well as a subplot involving a baby and a police cruiser, and also including boxer Mike Tyson. Filming took place in Nevada for 15 days, and during filming, the three main actors (Cooper, Helms, and Galifianakis) formed a real friendship.

The Hangover was released on June 5, 2009, and was a critical and commercial success. The film became the tenth-highest-grossing film of 2009, with a worldwide gross of over $467 million. The film won the Golden Globe Award for Best Motion Picture – Musical or Comedy, and received multiple other accolades. It became the highest-grossing R-rated comedy ever in the United States, surpassing a record previously held by Beverly Hills Cop for almost 25 years.

The film was followed by two sequels: The Hangover Part II (2011) and The Hangover Part III (2013). Both were commercial successes, but neither were well-received critically.