IU has been credited for writing over seventy-six songs over the course of her career, including music for her career as a solo artist, songs for other performers and various songs for film or drama. The first two songs for which she received songwriting credit were Yoo Seung-ho's "Believe in Love", which she featured on, for the compilation album Road for Hope, and "Alone in the Room" from her third extended play (EP) Real. Her first self-composition was "Hold My Hand" for The Greatest Love in 2011. Considering the fact that she wrote the song not for herself but at the behest of the drama's producers, IU found the experience difficult as she had to match the drama's cheerful tone. On her second studio album Last Fantasy, IU began to take more creative control over her music, writing six of the thirteen total tracks. Her following studio album Modern Times and its re-release Modern Times – Epilogue was also mostly written by IU and South Korean lyricist Kim Eana, who had previously written multiple tracks on Last Fantasy. IU wrote the lyrics to each track on her subsequent releases, Chat-Shire and Palette, with the exception of one song on the latter, and served as executive producer of both. The lyrics on her fifth EP Love Poem were entirely self-written, and she received composition credits on two tracks, "The Visitor" and "Blueming". IU wrote lyrics to all the tracks on her fifth studio album, Lilac. Additionally, she helped compose two of the album's singles, "Coin" and "Celebrity". IU has also written and performed multiple songs for various South Korean films and dramas, including The Producers, My Mister, Bel Ami and Hotel Del Luna. Sixteen of the tracks written by IU have been released as singles, with eleven of those reaching the top of the Gaon Digital Chart, including "Through the Night" and "Friday", the third and fifth best-selling singles in South Korea, with more than six million and five million sales, respectively.

According to The Korea Herald, consolation is one of the core themes explored in IU's music, most notably in "Through the Night" and "Dear Name", both songs from Palette.