A piano-based ballad song, it begins with a piano melody, followed with electric guitar lines, that accompanies slow-tempo drumming.

It appeared on the United States Billboard Hot Adult Contemporary Tracks chart and peaked at number 37, making it Coldplay's first appearance on that chart and helping establish moderate success at AC radio for future singles like "Viva la Vida", "Paradise", and "Every Teardrop Is a Waterfall".

[5][6] Coldplay released "The Hardest Part" in the US and UK on 3 April 2006 and in Japan on 24 May as the fourth single from X&Y.

"[12] Michael Hubbard of MusicOMH wrote: "'A Message' and 'The Hardest Part' sound like companion pieces, both big songs.

[14] Adrien Begrand from PopMatters wrote that "The Hardest Part" is a "pleasant slice of R.E.M.

"[15] Cameron Adams of the Herald Sun reported that the song sounded "like the Smiths meets REM.

"[16] David Cheal of The Daily Telegraph noted, "...'The Hardest Part' is lovely, straightforward, instantly accessible pop-rock".

[17] The music video for "The Hardest Part" was shot on 3 March 2006 in St. Petersburg, Florida.

[18] American actress Linda Dano, who played Felicia Gallant on the US soap opera Another World, is also featured.