"Day by Day" (Korean: 하루하루, Revised Romanization: Haru Haru) is a song by South Korean boy band Big Bang. Released through YG Entertainment on August 8, 2008 as the lead single for their third extended play Stand Up, it became one of their most popular singles and one of South Korea's best-selling songs, besides being regarded as one of the most influential singles to come out of Korean pop music. A Japanese version of the song was included in their third Greatest Hits album The Best of Big Bang (2011) and in the Japanese version of Alive (2012).

With the release of their first EP Always (2007), Big Bang began to incorporate a range of different genre into their music, moving away from their original hip hop roots. Stand Up continued their experimentation. The song was written by G-Dragon, with arrangements provided by Japanese DJ Daishi Dance. "Day by Day" was described as a combination of a dance song with a hip hop ballad, driven by a sentimental piano melody. The song also includes a synthesizer sound that sounds like an orchestra that feels its "empty spaces". The composition was noted for "weaving in and out of tempos seamlessly" which forced a "a sense of anxiety on the listener." "Day by Day" deepened the "Big Bang style" of music that had its breakthrough with "Lies." Tamar Herman of Billboard wrote about the track's composition:

The song begins simply enough with a piano tune and breathy vocals until T.O.P's deep bass rap explodes to transform it into a hip-hop ballad like none other. G-Dragon and T.O.P's contrasting rap tones play off one another to offer a sharp change from the passionate vocals of Daesung, Taeyang, and Seungri, while the orchestral accompaniment draws the whole thing to a symphonic peak. Nearly ten years after its release, 'Haru Haru' [sic] is still the epitome of Big Bang's sound.

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