This was the first of two Backstreet Boys albums that the band released as a quartet, as Kevin Richardson departed from the group in 2006 to pursue other interests.

Members of the Backstreet Boys themselves co-wrote five tracks on this album along with longtime songwriter Jeremy Carpenter from Kentucky, including "Intro".

[5] AJ McLean revealed to MTV News at the Moto 8 Party in Hollywood that "The boys and I are finishing up the new record, coming out at the top of the year."

[10] The third song and lead single "Inconsolable" was produced by Emanuel Kiriakou and written by Emmanuel, Lindy Robbins and Jess Cates.

[12] "One in a Million", which conjures memories of the stuttering eight-note beats of "Larger Than Life",[14] has elements of hip hop and reggae.

[9] The eighth track "Panic" add rock elements to the group's sound[8] and is another more adventurous outing; beginning with a dubby drum and bass rhythm.

[13] "Love Will Keep You Up All Night" was classified as a "montage of almost every love-song ever written and with a chorus reminiscent of Aerosmith's 'I Don't Want to Miss a Thing'.

Commencing with only a piano, the song then adds a live drum beat and orchestral instruments, followed by an electric guitar solo and atmospheric harmonies.

[16] Bill Lamb from About.com gave the album 4 out of 5 stars, praising "the incomparably lush harmonies and instantly pleasing melodies" and labeling the songs "radio friendly.

"[8] Lamb also commented that the band "have settled into a comfortable place as an adult vocal group" and named the album "a pure pop pleasure.

"[8] Tammy LaGorce from Amazon.com judged that the album "boasts Super Glue-strength harmonies and an overall tightness of sound" and that "the music is more thoughtful and mature than ever.

"[15] Simon Vozick-Levinson of Entertainment Weekly judged that "They come close to recreating the sheer euphoria of their biggest pre-2YK smashes on many of the up-tempo dance-pop numbers that follow.

"[10] Matt O'Leary from Virgin Media opined that "While there is nothing to match their crowning glory – karaoke classic "I Want It That Way" – this is a largely inoffensive collection of pop hits.

"[11] Lauren Murphy from Entertainment.ie called Unbreakable "far from the perfect pop album - in fact, it's rather boring for the most part - but it does confirm that Backstreet Boys aren't quite ready for the scrap heap just yet.

"[13] Christian Hoard from Rolling Stone gave the album 2 out of 5 stars, summarizing that "Unbreakable makes small nods to adult pop, peppering the processed music with tasteful piano and light guitar riffs and keeping bright, danceable grooves to a minimum.

In its second week, the album maintained its number one position beating all of the domestic Japanese competition, a remarkable feat for a foreign band.