Stylistically, it builds heavily around the band's trademark power chords and guitar riffs, and also employs a church organ to achieve a funereal atmosphere.

"Not Now" shares writing credits between the band's three members: guitarist Tom DeLonge, bassist Mark Hoppus, and drummer Travis Barker.

An outtake from the band's untitled 2003 album, "Not Now" had previously been released as a standalone single for iTunes and elsewhere prior to its inclusion on Greatest Hits.

"[4] The song's guitar riffs were compared to the Descendents by journalist Joe Shooman, and are interspersed by verses containing a church organ.

[5] Matt Mitchell of Paste also noted the similarities to the Descendents, dubbing the song "a tight, riff-heavy, pop-punk performance".

called it among the band's best, observing, "You can feel and hear the friction in the track [...] DeLonge appears to be pushing a separate sonic agenda which he eventually realized with Angels & Airwaves, while vocalist/bassist Mark Hoppus and drummer Travis Barker are busy reinforcing the trademark Blink sound.