"Yesterday" is a song by the English rock band the Beatles, written by Paul McCartney and credited to Lennon–McCartney. It was first released on the album Help! in August 1965, except in the United States, where it was issued as a single in September. The song reached number one on the US Billboard Hot 100 chart. It subsequently appeared on the UK EP Yesterday in March 1966 and made its US album debut on Yesterday and Today, in June 1966.

McCartney's vocal and acoustic guitar, together with a string quartet, was essentially the band's first solo performance. It remains popular today and, with 2,200 cover versions, is one of the most covered songs in the history of recorded music. "Yesterday" was voted the best song of the 20th century in a 1999 BBC Radio 2 poll of music experts and listeners and was also voted the No. 1 pop song of all time by MTV and Rolling Stone magazine the following year. In 1997, the song was inducted into the Grammy Hall of Fame. Broadcast Music Incorporated (BMI) asserts that it was performed over seven million times in the 20th century.

"Yesterday" is often interpreted as a melancholic ballad about lost love. Paul McCartney later suggested that the lyric "I said something wrong" may have been inspired not by romance but by a childhood memory of mocking his mother for sounding "posh". McCartney is the only member of the Beatles to appear on the track. The final recording was so different from other works by the Beatles that the band members vetoed the song's release as a single in the United Kingdom. However, other artists quickly recorded versions of it for single release. The Beatles' recording was issued in the U.K. as a single in 1976 and peaked at number 8 on the UK Singles Chart.

According to biographers of Paul McCartney and the Beatles, the entire melody came to McCartney in a dream one night in his room at the Wimpole Street home of his then-girlfriend Jane Asher and her family. Upon waking, he hurried to a piano and played the tune to avoid forgetting it. Initially he was concerned, though, that he had subconsciously plagiarised someone else's work; as he put it: "For about a month I went round to people in the music business and asked them whether they had ever heard it before. Eventually it became like handing something in to the police. I thought if no one claimed it after a few weeks then I could have it."

Upon being convinced that he had not copied the melody, McCartney began writing lyrics to suit it. As John Lennon and McCartney were known to do at the time, a substitute working lyric, titled "Scrambled Eggs" (the working opening verse was "Scrambled eggs/Oh my baby how I love your legs/Not as much as I love scrambled eggs"), was used for the song until something more suitable was written.

During the shooting of Help!, a piano was placed on one of the stages where filming was being conducted, and McCartney took advantage of this opportunity to tinker with the song. This eventually greatly annoyed the director Richard Lester, who lost his temper, telling McCartney to finish writing the song or he would have the piano removed. The patience of the other Beatles was also tested by McCartney's work in progress; George Harrison summed this up when he said: "Blimey, he's always talking about that song. You'd think he was Beethoven or somebody!"

McCartney originally claimed he had written "Yesterday" during the Beatles' tour of France in 1964; however, the song was not released until the summer of 1965. During the intervening time, the Beatles released two albums, A Hard Day's Night and Beatles for Sale, each of which could have included "Yesterday". Although McCartney has never elaborated on his claims, a delay may have been due to a disagreement between McCartney and George Martin regarding the song's arrangement or the opinion of the other Beatles who felt it did not suit their image.

Lennon later indicated that the song had been around for a while before: