"A Hard Day's Night" is a 1964 hit song written by John Lennon and credited (as all their songs were) to John Lennon and Paul McCartney, performed by English band The Beatles and produced by George Martin.
It was released on the movie soundtrack of the same name. It was later released as a single, with "Things We Said Today" as it's b-side.
The song featured prominently on the soundtrack to The Beatles' first feature film, A Hard Day's Night, and was on their album of the same name. The song topped the charts in both the United Kingdom and United States when it was released as a single.
Featuring a prominent and unique opening chord, the song's success provided the first inkling that The Beatles were not the one-hit wonder some had suggested when they first came to America