Written by Diane Warren and produced by David Hodges, it was released on August 20, 2013.

[1] Demi Lovato was announced as part of The Mortal Instruments: City of Bones soundtrack in July 2013.

Hollywood Life correspondent Tierney McAfee labelled "Heart by Heart" the best love song of the summer "by far", citing the dynamic production and Demi's emotive vocals, which McAfee described as "pack[ing] a big emotional punch".

[1] The Huffington Post called the song "hauntingly beautiful" and praised the "powerful" piano-based instrumentation.

[7] Sam Lansky of Idolator lauded Lovato's "absolutely monster vocals" on the "soaring power ballad" and felt the song was anthemic, drawing comparisons to other Warren songs.