Sir George Henry Martin was an English record producer, arranger, composer, conductor, and musician. He was commonly referred to as the "fifth Beatle" because of his extensive involvement in each of the Beatles' original albums. Martin's formal musical expertise and interest in novel recording practices facilitated the group's rudimentary musical education and desire for new musical sounds to record. Most of their orchestral and string arrangements were written by Martin, and he played piano or keyboards on a number of their records. Their collaborations resulted in popular, highly acclaimed records with innovative sounds, such as the 1967 album Sgt. Pepper's Lonely Hearts Club Band—the first rock album to win a Grammy Award for Album of the Year.

The fifth Beatle is an informal title that has been applied to people who were at one point a member of the Beatles or who had a strong association with John Lennon, Paul McCartney, George Harrison, and Ringo Starr. The "fifth Beatle" claims first appeared in the press immediately upon the band's rise to global fame in 1963–64. The members have offered their own views as to who should be described with the title:McCartney said on two occasions that "if anyone was the fifth Beatle", it was manager Brian Epstein and producer George Martin. Harrison stated at the Beatles' 1988 induction into the Rock and Roll Hall of Fame that there were only two "fifth Beatles": Derek Taylor, the Beatles' public relations manager, and Neil Aspinall, their road manager-turned-business-executive.