[]-X-[] [1999] エックス

X (Japanese: エックス Hepburn: Ekkusu), also known as X/1999, is a Japanese shōjo manga series created by Clamp, a creative team made up by Satsuki Igarashi, Nanase Ohkawa, Tsubaki Nekoi, and Mokona. It premiered in Monthly Asuka's May 1992 issue and ran there until the magazine's editors showed concern in the increasingly violent stories. The series went on long-term hiatus in March 2003 and has yet to be concluded. The story takes place at the end of days, in the year 1999. The series follows Kamui Shirō, a young esper who returns home to Tokyo after a 6-year absence to face his destiny as the one who will determine humanity's fate.

Kadokawa Shoten collected and published the individual chapters in 18 tankōbon volumes, with five chapters published in the book "All about Clamp". All but several final chapters have been published. It has been adapted into a series of audio dramas, a 1996 animated feature film and a television series in 2001 with the two latter produced by Madhouse. Viz Media published all 18 volumes in North America, while the film and the TV series have released in DVDs by Manga Entertainment and Geneon, respectively.

The series is a foray into apocalyptic fiction; it combines elements from various end of the world scenarios, both secular and religious, with its own mythos. Its many themes include exploring the personality of mankind and their relationship with others and external conflicts, such as the impending Armageddon. It has also garnered mostly positive reviews, despite its lack of an ending.