Named after a form of Buddhist meditation, Zazen Boys first appeared as a side project of Japanese musician Mukai Shutoku, the guitarist/vocalist of alt-rock band Number Girl, as an opportunity for Mukai to express some of his more diverse and experimental musical urges at a time when his other band was riding the crest of its breakthrough into the mainstream. After Number Girl's demise he revived the project, taking them to new levels of critical and commercial success and proving a critical link between mainstream and underground music in Japan.

The original 2002 lineup of Zazen Boys featured Mukai backed by the experimental hip-hop/funk band 54-71, although he undertook his immediate post-Number Girl activities under his own name and with a variety of musicians, including avant-garde jazz-punk band Panic Smile (early contemporaries of Mukai from his hometown of Fukuoka) and jazz musician Kikuchi Naruyoshi. In contrast to the Pixies-style alt-rock that had made Number Girl famous, these early shows mixed jazz, funk, rock, and hip-hop, and it was this collision of genres that formed the basis for what was to become the new Zazen Boys.

Establishing the new lineup around ex-Number Girl drummer Inazawa Ahito, former Art-School bass player Hinata Hidekazu, and guitarist Yoshikane Sou from Kicking the Lion, Mukai started work on a self-titled debut album, Zazen Boys, which was released in 2004 and quickly followed up by a second, entitled Zazen Boys, Vol. 2, during the same year. Both albums drew on and developed ideas mixing funk, rock, and elements of traditional Japanese culture partially developed in Number Girl's final album, Num-Heavy Metallic, as well as the more experimental work of the post-Number Girl live shows. The punk influences of his previous work gave way to a new interest in heavy, crashing guitars reminiscent of Led Zeppelin. Other characteristic features are obvious dub, hip-hop, and soul influences; frequent changes in time signature; and Mukai's rapid-fire half-spoken vocals interspersed with sudden bursts of falsetto.

In 2005 Inazawa departed to form the more new wave-influenced Vola and the Oriental Machine, and he was replaced by Matshshita Atsushi for the Himitsu Girl's Top Secret EP and 2006's uncompromising Zazen Boys, Vol. 3, which saw the band pushing its experimental tendencies still further. Another departure came in 2007 -- this time it was Hinata, subsequently replaced by Yoshida Ichiro of Nine Days Wonder. This was followed by the single I Don't Wanna Be with You, which bemused fans by overlaying a minimalist funk backdrop with a glossy, synth-led 1980s sheen. In spring 2008 the band decamped to the United States to record new material. Zazen Boys, Vol. 4 was released in September of that year. ~ Ian Martin, Rovi