"Good-Bye" (Japanese: グッドバイ, Hepburn: Guddobai^?) (Japanese pronunciation: [gɯd:obai]) is a song by Japanese band Sakanaction. It was released as a single in January 2014, as a double A-side single with the song "Eureka". A rock ballad primarily based around non-electronic instruments, the song was composed by the band's vocalist Ichiro Yamaguchi about his mental state while physically unwell and mentally frustrated, after being unable to finish writing the song "Sayonara wa Emotion". In January 2014, a remix of the song was used in the NHK documentary program Next World: Watashi-tachi no Mirai, later to be included on the band's compilation album Natsukashii Tsuki wa Atarashii Tsuki: Coupling & Remix Works (2015).

The physical single debuted at number two on Oricon's weekly singles chart, while "Good-Bye" outperformed "Eureka" on the Billboard Japan Hot 100, also reaching number two. Critics received the song well, praising the song for its simple band sound that developed with the additional of guitar feedback, and believed that the song was an expression of a musician reaffirming their identity and their desire to continue into uncharted musical territory.

In March 2013, Sakanaction released their sixth studio album Sakanaction. The album was a result of the band's vocalist and songwriter Ichiro Yamaguchi felt a new resolution to create music that would resonate with a general pop music audience, and featured several songs with high profile commercial tie-ups. The first of these was the band's single "Boku to Hana", released in May 2012, which was used as the theme song for the Tsuyoshi Kusanagi drama 37-sai de Isha ni Natta Boku: Kenshui Junjō Monogatari, Three months later, the band followed this with the single "Yoru no Odoriko", a song that had been featured in commercials for the design school Mode Gakuen from April 2013. A month and a half before the album's release, the band released the single "Music", a song used as the theme song for the Yōsuke Eguchi-starring Fuji Television drama Dinner. Another song found on the album, "Aoi", was commissioned by the Japanese national broadcaster NHK for a theme song for them to use for their 2013 broadcast of soccer events. The band wanted to balance these songs created for a wide audience with songs that they personally wanted to create, so gave the album a Möbius strip theme to illustrate this. The album debuted at number one on Oricon's weekly albums chart, after selling 83,000 copies. This was a record for the band, both in terms of the number of copies that they had sold in one week of a release, as well as the fact that they had never reached number one on an Oricon chart before. The release is currently the band's most successful album, in terms of physical copies sold.

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