Lavigne and production team the Matrix (Lauren Christy, Scott Spock, and Graham Edwards) are credited as writers.

"Complicated" received generally positive reviews from music critics, who praised the song as infectious pop rock.

The song also spent a consecutive sixteen weeks at number one on the Billboard Adult Top 40 chart, breaking a record previously held by Natalie Imbruglia's "Torn".

After being signed to Arista Records, Lavigne moved to New York and began working on her debut album, Let Go, collaborating with a host of prime songwriters and producers.

[2] Lavigne relocated to Los Angeles, where she collaborated with songwriter-producer Clif Magness, who gave her ample creative control in the writing process.

Lavigne and Magness wrote "Losing Grip" and "Unwanted", songs that she deemed reflective of her vision for the entire album.

[3] However, Arista was not thrilled with the heavy-guitar laden songs that Lavigne was writing, prompting the label to look for other producers to match their demands.

[5] According to member Lauren Christy, they had been listening to Lavigne's early songs and felt they contained "a Faith Hill kind of vibe".

[5] Composed in the key of F major, "Complicated" is a pop rock[6][7][8] song about how people can feign or pretend in front of others.

[12] As of 2023, "Complicated" ranks as Lavigne's most popular song on several platforms, including Spotify and several other pop rock music list sites.

David Browne of Entertainment Weekly gave the song a B− and said "Avril Lavigne's not kidding about that title, she's the epitome of the blossoming anti-Britney movement".

[13] Christina Saraceno of AllMusic described "Complicated" as "a gem of a pop/rock tune with a killer chorus" and noted similarities between it and Pink's song "Don't Let Me Get Me".

[14] In a review of Lavigne's second album, Under My Skin, David Browne of Entertainment Weekly noted that "'Complicated' felt like strung-together bits of Morissette songs".

[22] In the United Kingdom, "Complicated" peaked at number three on the UK Singles Chart and was certified 2× platinum with more than 1,200,000 copies sold and streamed.

[27] "Complicated" was certified gold in Austria, Belgium, Italy, Japan, Sweden and Switzerland, and platinum in Brazil and Norway.