Glay (stylized in all caps) is a Japanese rock band formed in Hakodate in 1988. The core four members, vocalist Teru, guitarists Takuro and Hisashi, and bassist Jiro, have been together since 1992. Primarily composing songs in the rock and pop genres, they have also arranged songs using elements from a variety of other genres, including progressive rock, punk, gothic rock, electronic, R&B, folk, gospel, reggae, and ska. Originally a visual kei band, the group slowly shifted to less dramatic attire through the years. With five million copies sold, Glay's 1997 compilation album Review is the fifth best-selling album of all time in Japan. Their July 1999 concert "Expo '99" at Makuhari Messe was attended by 200,000 people, making it the largest-ticketed concert ever held by a single act at the time. As of 2008, Glay had sold an estimated 51 million records; 28 million singles and 23 million albums, making them one of the top ten best-selling artists of all time in Japan.

Glay formed in 1988 as a high school band when Takuro asked Teru, a schoolmate, to play the drums. They found a bassist but had difficulty finding a vocalist. When Teru made a tape of his singing and gave it to Takuro he was immediately recruited for the part, leaving the drums part to be filled by another person. On the search for a second guitarist, Hisashi was asked to join but declined the offer, as he was already part of a locally well-known heavy punk/rock band called Ari, which better suited his taste in music. Hisashi eventually accepted Takuro's offer and became Glay's lead guitarist after Ari disbanded in 1989. Officially, the name "Glay" was coined by Takuro and is a deliberate misspelling of the word "gray" to represent the style of music they wanted to play: a mixture between rock (black) and pop (white). However, Hisashi confirmed in an interview with Natalie that the name came from Jun Gray, a member of the punk rock band Kenzi & The Trips that he and Takuro had read about in the magazine Takarajima.

By the time of Takuro and Teru's graduation, Glay were enjoying some popularity in their hometown of Hakodate and were playing full live houses. Following Hisashi's high school graduation in 1990, the three moved to Tokyo to try to further expand their musical career. The bassist and the drummer chose to stay in Hakodate. In Tokyo, they found a completely different situation: although they had been relatively popular in their hometown, it was not easy to begin a career in Tokyo. Their concerts attracted few people, and sometimes none at all, and many live houses would not accept them because they did not fit well into either rock or pop categories. The band had to conciliate their music career to part-time jobs and faced financial problems during their first years in Tokyo.

During this time, members (particularly drummers) were constantly joining and leaving the band. When their bassist quit, Takuro knew that Jiro, who was also from Hakodate and had played with the indie band Pierrot (different from the now disbanded Pierrot, that enjoyed mainstream popularity), had moved to Tokyo and invited him to join Glay. He too, declined the offer, insisting that he was already heading in the right direction for himself. It wasn't until Takuro asked him to play at just one show to fill in for their missing bassist that he decided to go. Following that show, Jiro continued to receive invitations to play with Glay, and eventually he became Glay's official bass player, finalizing the official four member lineup in August 1992.

They promoted the band by handing out flyers on the street and giving out demo tapes. Eventually they became better known on the Tokyo live house circuit and began drawing larger crowds. Eventually, hide of X Japan gave one of Glay's demos to his bandmate Yoshiki. During an October 1993 show, Yoshiki and his entourage came to watch and offered the band a contract to his record label Extasy Records. Their debut single, "Rain", and their first album, Hai to Diamond, were both released on May 25, 1994.

After the time of their debut, Glay steadily became more popular. Their 4th single "Freeze My Love" made it onto the Oricon Top 20, debuting at No. 19. Two months later their second album, Speed Pop, peaked at No. 8. It was their first album released on Platinum Records, a division of PolyGram that Yoshiki founded that same year. Their first No. 1 hit was in 1996 with their third album Beat Out!, which was marked as a major turning point in Glay's career. Released later that same year, the fourth album Beloved was their first album to sell over 1 million copies. In September 1996, Glay played the final concert of the Beat Out! Reprise Tour at Nippon Budokan, their first concert at the venue, which is stated by them to have been their biggest dream coming true. In August 1997, their 12th single "However" remained at the top of the charts for two weeks in a row and, after it was displaced by Zard's single "Eien" for one week, it re-appeared at the top position for another three weeks.

In the period between 1997 and 2000, Glay produced six million-selling singles, being the third artist with most million-selling singles of all time.^{[citation needed]} Their 13th single "Yuuwaku" topped Oricon yearly single ranking in 1998. In 1999, their 16th single "Winter, Again" was number two of the year and won the 41st Japan Record Awards "Grand Prix" award, which is considered the most important Japanese music award. Their albums were also massively successful; both Pure Soul (1998) and Heavy Gauge (1999), along with the compilation album Drive - Glay Complete Best (2000), sold more than two million copies each.

The impact of their popularity was such that, in 1998, Japan's telephone service was temporarily put out of order due to the mass number of fans trying to reserve tickets to their upcoming tour. The 1998 Winter Olympics were being held, and press coverage of the event became chaotic because of the incident.