It sold three million copies in the United States, ranking the album number 149 on the Billboard 200 decade-end chart.

[1] Because of the album's darker, heavier, more aggressive vibe reminiscent of post-grunge and more melodic rock songs, it received generally positive reception from critics at the point of considering it as one of the classic albums that defined pop-punk music in the early 00s (despite it not being usually classified as such),[2] and also one of the works that anticipated the emotional intensity and theatrical aesthetics of emo pop music in the mainstream.

[3][4] On March 18, 2013, Under My Skin was re-released as a double-disc set paired with her debut studio album, Let Go, which was released under Arista Records.

Under My Skin has sold over 10 million copies worldwide and is the fifth best selling album of the 21st century by a Canadian artist.

Having no plans of working with producers or professional writers,[23] Lavigne wrote much of the album with Canadian singer-songwriter Chantal Kreviazuk, with whom she had developed a friendship in the summer of 2003.

Kreviazuk invited Lavigne to continue working in a Malibu, California house she shared with Maida, which contained a recording studio.

[26] Lavigne also co-wrote one track, "Nobody's Home", with Ben Moody, formerly of Evanescence, and the rest with her guitarist Evan Taubenfeld.

According to Metacritic, Under My Skin received an average rating of 65, reflecting a generally positive reception from critics.[35].

David Browne of Entertainment Weekly suggested that in the album, "Lavigne has become even more, well, complicated", noting she "sounds more burdened".

"[33] Sal Cinquemani of Slant magazine noted that Lavigne's sound was now much heavier and darker and compared her to Amy Lee of Evanescence, as did Browne.

[33] Carly Carioli of Blender magazine also agreed, stating "she has deepened and darkened her sound without sacrificing her platinum-plated melodies".

[34] Kelefa Sanneh of Rolling Stone praised Lavigne's vocals, "blankness is what makes her best songs so irresistible.

[39] Musically the album's sound is compared to "that of crunching punk guitars playing mighty power chords, all mixed with the same flawless elan that has characterized pop-punk ever since Green Day dropped Dookie" says Tim O'Neil of PopMatters.

[41] On a more mixed note, Stephen Thomas Erlewine of AllMusic wrote that the album is "a bit awkward, sometimes sounding tentative and unsure, sometimes clicking and surging on Avril's attitude and ambition."

Accompanied by her guitarist Evan Taubenfeld, Lavigne performed a short live acoustic version of five songs from the album.