CK5 is a compilation album by Japanese contemporary R&B singer Crystal Kay, released on June 30, 2004. It commemorates the five years since her debut as a musician in 1999. On the same day, CK 99—04 Music Clips, a video album compiling her music videos was released.

In 2001, Crystal Kay started to collaborate with the hip-hop/R&B group M-Flo, with Verbal featured on her single "Ex-boyfriend", and Taku Takahashi producing her singles "Hard to Say" (2002) and "Boyfriend (Part II)" (2003). In 2003, Crystal Kay collaborated on with them on two singles, "Reeewind!" and "I Like It". November saw the release of an Asia-wide English language album Crystal Kay (released as Natural in Japan) and her fourth studio album 4 Real.

In May 2004, Crystal Kay released "Motherland", a ballad written for the anime Fullmetal Alchemist, which became her second top ten single. In June, she announced on her official website that she had graduated high school.

In April 2004, Crystal Kay's official site ran a poll to find out what the most popular songs of Crystal Kay's were for her fans. The resulting top 10 songs became the basis for the track list of CK5. Three songs that were not on the list made it onto the album: "Motherland", which was the preceding single for the album, "Over the Rainbow", which was used in commercials for Tokyo Mode that featured Crystal Kay personally, and "Lead Me to the End", which had been used as a theme song for a Fuji Television new years drama, Guchi 2.

The album compiles songs from her previously released studio albums, as well the single "Motherland". Half of the album is taken from her fourth studio 4 Real (2003) which had been released seven months ago, as well as two songs from Almost Seventeen (2002) ("Think of U" and "Hard to Say"), three from 637: Always and Forever (2001) ("Ex-Boyfriend", "Tsuki no Nai Yoru, Michi no Nai Basho" and "Lost Child") and one from C.L.L Crystal Lover Light (2000), her debut single "Eternal Memories" (1999).

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