"So Yesterday" is a song by American singer Hilary Duff for her second studio album, Metamorphosis (2003).

It was written and produced by The Matrix (a production team consisting of Lauren Christy, Scott Spock and Graham Edwards), with additional writing from Charlie Midnight.

The song is influenced by pop rock music, and the lyrics chronicle the protagonist getting over a breakup with her boyfriend, declaring him as "so yesterday".

The song was accompanied by a music video, directed by Chris Applebaum, which showed Duff playing a prank on her ex-boyfriend.

Although Hilary Duff recorded songs like "I Can't Wait", "Why Not" and "What Dreams Are Made Of", which received substantial airplay on Radio Disney,[1] executives at Buena Vista Music Group, Duff's record label, planned to help her reach a more mature audience.

"[3] Bob Cavallo, chairman of Buena Vista Records, said that the song was "more mature than the past work that she's done.

[11] She said her mother Susan Duff instilled the notion of inner strength in her, and that she wanted the music she makes to be positive.

[11] She stressed the importance of strong songs that are not all about how "a boy broke your heart and you're never going to get on your feet again", saying it was good for girls to hear.

Stephen Thomas Erlewine of AllMusic gave a positive review and commented that the song sounded natural coming from a teenager like Duff.

[14] Similarly, Spence D. of IGN Music was mixed in his review stating, "[it's] like bubblegum ice cream: smooth and sweet at first listen, but the flavor eventually wears off, leaving somewhat generic pop anthems that will most likely be forgotten by the aforementioned throngs of young girl worshippers as they age along with Duff.

[29] According to Disney Music Group chairman Bob Cavallo, care was taken to see that the video would not be offensive to Duff's young fans and parents, but would work for MTV.

She spends several days taking snapshots of strangers wearing the T-shirt and anonymously sneaking them into his mailbox.

His bewilderment turns to dismay when he receives the final package returning the garment along with a picture of Duff smiling, looking back, and wearing a shirt that declares, "you're so yesterday".

The live acoustic performance at Sessions@AOL was taped and is featured on Duff's video compilation All Access Pass,[34] and the performance at the Metamorphosis tour is featured on Duff's 2004 live video compilation The Girl Can Rock.