"As Time Goes By" is a song written by Herman Hupfeld in 1931. It became famous when it was featured in the 1942 Warner Bros. film Casablanca performed by Dooley Wilson as Sam. The song was voted No. 2 on the AFI's 100 Years...100 Songs special, commemorating the best songs in film only surpassed by "Over the Rainbow" by Judy Garland). The song has since become the signature tune of Warner Bros. and used as such in the production logos at the beginning of many Warner Bros. films since 1999, as well as the closing logos to most Warner Bros. Television shows since 2003. It was also the title and theme song of the 1990s British romantic comedy series As Time Goes By.

The AFI listed it among its "top 100" movie songs. National Public Radio included it in its "NPR 100", a 1999 list of the most important American musical works of the 20th century as compiled by NPR's music editors.The song is a popular reflection of nostalgia and often used in films and series reflecting this feeling

Herman Hupfeld wrote "As Time Goes By" for the 1931 Broadway musical Everybody's Welcome which opened on October 31, 1931. In the original show, it was sung by Frances Williams. It was first recorded by Rudy Vallée on July 25, 1931 for Brunswick Records, then also by Jacques Renard and his Orchestra and Fred Rich. In 1932, Binnie Hale recorded the song. In terms of popularity at the time, it was a modest hit. Elisabeth Welch included the song in her cabaret act soon after it was released.

The song was re-introduced in the 1942 film Casablanca where it was sung by Sam, portrayed by Dooley Wilson; Sam's piano accompaniment was played by a studio pianist, Jean Vincent Plummer. The melody is heard throughout the film as a leitmotif Wilson was unable to make a commercial recording of the song at the time due to the 1942–44 musicians' strike. Unable to record new versions of the song, RCA Victor reissued the 1931 recording by Rudy Vallée which became a number one hit, eleven years after it was originally released. Brunswick also reissued the 1931 Jacques Renard recording.

Hupfeld lived his whole life in Montclair, New Jersey, and was a regular customer at the Robin Hood Inn, a tavern built in 1922 on Valley Road, then part of Upper Montclair. He spent many hours at the piano and wrote several of his songs in this tavern, now the Valley Regency. A plaque on the second floor of the Valley Regency Catering Facility in Clifton, New Jersey, commemorates the song. He wrote over one hundred songs, including "Let's Put Out the Lights and Go to Sleep," and the popular Great Depression song "Are You Making Any Money?

The original song in the film as sung and played by "Sam" was recorded in D-flat major, but it has since been played in several keys, commonly C major, but also B-flat major, as in Frank Sinatra's recording, and other keys including A major and E-flat major, the key in which the song was originally published.

In Casablanca, Wilson sings the chorus ("You must remember this..."), but does not sing the verse ("This day and age we're living in..."). The omitted lines refer to Albert Einstein.