Written by Pink, Max Martin and Lukasz "Dr. Luke" Gottwald, the song was originally released on May 8, 2006, by the LaFace label to radio as the album's second single, but was only a moderate success.

Upon its release, "Who Knew" was well received by music critics, who acclaimed it as a solid song on the album and lauded the lyrical content.

The accompanying music video to the song was directed by Dragon and was released in May 2006, where it shows Pink singing at a county fair, with a storyline about two young lovers who end up being separated after the man begins using drugs.

The NY Daily quotes Pink as writing the song to a male friend (not a boyfriend) she lost to drug abuse.

"[9] Sal Cinquemani from Slant Magazine wrote that "Songs like 'Who Knew' and 'Long Way To Happy', though good, sound more hair metal than punk", while also noting that it "follows the steps of Clarkson's recent hits.

"[10] Billboard magazine, in its second review of the song (to accompany the U.S. re-release), called it "another cool, smart, plenty passionate melodic jewel about the death of a friend and/or friendship, with an instantly identifiable, quick-to-the-point, singalong track—maintaining Pink's stance as the smartest female millennium-era singer/songwriter, dazzling in star quality as alterna-pop's premier princess.

Billboard magazine credited the poor performance of the single on radio with significantly reducing momentum of initial sales of I'm Not Dead.

It was re-released to mainstream radio in the United States on June 26, 2007,[18] although it had already been receiving minor airplay on CHR/top 40 and hot AC/adult top 40 stations earlier in the month.

[20] The song is Pink's longest chart runner on the Billboard Hot 100, spending 36 weeks before dropping out (a feat that would later be matched by her 2013 hit, "Just Give Me a Reason" featuring Nate Ruess).

It was filmed in the weekend beginning April 15, 2006, in Los Angeles, United States,[24] and it was released to the internet and to UK music channels in early May 2006.

"Who Knew" was voted number one on TRL Germany twenty times, allowing Pink to earn the "Golden Tape".

[26] It features Pink wearing a light green dress and a young couple visiting a fairground and going on the rides, and at one point the boy puts a necklace on the girl.

The final scenes depict flashback sequences showing the couple on several carnival rides together and both Pink and the lovers submerged underwater.