"Sgt. Pepper" is regarded by musicologists as an early concept album that advanced the roles of sound composition, extended form, psychedelic imagery, record sleeves, and the producer in popular music. The album had an immediate cross-generational impact and was associated with numerous touchstones of the era's youth culture, such as fashion, drugs, mysticism, and a sense of optimism and empowerment. It is considered one of the first art rock LPs, a progenitor to progressive rock, and the start of the album era. In 1968, it won four Grammy Awards, including Album of the Year, the first rock LP to receive this honour; in 2003 it was inducted into the National Recording Registry by the Library of Congress. It has topped several critics' and listeners' polls for the best album of all time, including those published by "Rolling Stone" magazine and in the book "All Time Top 1000 Albums", and the UK's "Music of the Millennium" poll. It remains one of the best-selling albums of all time and the UK's best-selling studio album, with more than 32 million copies sold worldwide as of 2011.