The Beatles' 'Sgt. Pepper' album celebrates its 50th anniversary today

— -- It was 50 years ago today that "Sgt. Pepper's Lonely Hearts Club Band" got its official release in the U.K.

The Beatles' psychedelic masterpiece -- which hit U.S. stores a day later -- went where no album had gone before, combining such influences as classical, big band and Indian music with various experimental sounds and studio techniques.

Beatles expert Bruce Spizer tells ABC News that perhaps the most important achievement of "Sgt. Pepper," which Rolling Stone chose as its greatest album of all time, was that "it elevated rock music to an art form," while also being "a great listening experience."

As Spizer suggests, many fans may consider "Rubber Soul" to be The Beatles' best collection of songs, "Revolver" to be the band's best overall album and "Abbey Road" or the White Album to be their favorite album by the Fab Four. But he feels "Sgt. Pepper" is undeniably the group's "most important album."

The concept of "Sgt. Pepper," devised by Paul McCartney, recast The Beatles as an alter-ego group. Launching the album was the McCartney-penned title track, which introduced the mythical band, while a reprise of the song brought the record up to its unforgettable finale, the epic "A Day in the Life."

Beyond the innovative and captivating music, "Sgt. Pepper" features the iconic cover collage with The Beatles surrounded by cutouts of dozens of historic figures, celebrities and musicians. The album also was among the first to include printed song lyrics.

Following its release, "Sgt. Pepper" spent 15 consecutive weeks at No. 1 on the Billboard 200 chart. The album has gone on to sell more than 11 million copies in the U.S. and more than 5 million in the U.K.

Check out a new fan Q&A with McCartney focusing on Sgt. Pepper at PaulMcCartney.com.