"U" is a song by South Korean pop group Super Junior, and is the group's follow-up release to their debut album Twins. The single topped CD sales charts during the first several weeks after its release on June 7, 2006. "U" peaked at #1 on the monthly MIAK K-pop album charts and sold 91,416 copies by 2008. The song won a total of five music recognitions, being Super Junior's most successful single in the music charts until the release of "Sorry, Sorry" in March 2009. Promotional performances for the title song, "U", did not complete until September 2006. The last general performance for the song was in August 2007 in Tokyo, Japan.

Since the Korean release of "U" in 2006, there has been two more releases of "U" in two different languages, Mandarin and Japanese. The Mandarin version of the single's title song "U" was released in a special Taiwan edition on June 15, 2007. On July 9, 2008, "U / Twins" was released in Japan, which included a Japanese version of "U" in the limited edition release.

Super Junior-M, Super Junior's Mandopop subgroup, also released a cover of "U" on April 8, 2008, marking it the second Mandarin version and third remake of "U". In this version, however, the lyrics were changed completely from the first Mandarin version. Also, new incorporations of a new jazz swing style with new acoustic guitar arrangements and an added hip-hop dance bridge and violin bridge were added to make the song suitable for the Chinese language while keeping close to the original Korean pop vibe.

Prior to "U", Super Junior was known for their member line-up, a concept very similar to Japanese girl group Morning Musume. Certain members of Super Junior's first generation were to be switched out and new members were to be added, which would form a second Super Junior generation. However, after the addition of a thirteenth member, Kyuhyun, the group ended their status as a project group and began to release productions as a group with permanent status. "U" is Kyuhyun's debut single.

"U" was made available for download on May 25, 2006 via Super Junior's official website. The second track from the single, "Endless Moment", became available for download four days later. "U" had over 400 thousand downloads within five hours of release and ultimately exceeded 1.7 million downloads, crashing the server.

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