Hilary Erhard Duff (born September 28, 1987) is an American actress, singer, author and businesswoman. She is the recipient of various accolades, including seven Kids' Choice Awards, four Teen Choice Awards and two Young Artist Awards, and nominations for two People's Choice Awards.

Duff began her acting career at a young age, quickly being labeled a teen idol as the title character in the Disney Channel comedy series Lizzie McGuire (2001–2004) and in the film based on the series, The Lizzie McGuire Movie (2003). Thereafter, she appeared in numerous mainstream films such as Cadet Kelly (2002), Agent Cody Banks (2003), Cheaper by the Dozen (2003), and A Cinderella Story (2004). She later appeared in independent films playing a wider range of adult-themed roles, such as War, Inc. (2008), According to Greta (2009), Bloodworth (2011), and The Haunting of Sharon Tate (2019). Duff starred as Kelsey Peters in TV Land's longest-running original series Younger (2015–2021), and produced and starred as Sophie Tompkins in the Hulu sitcom How I Met Your Father (2022–2023).

Musically, Duff first came to prominence after releasing her debut studio album, the Christmas-themed Santa Claus Lane (2002), through Buena Vista Records. Her second album, Metamorphosis (2003), was hugely successful, topping the Billboard 200 chart and earning a 4× Platinum certification by the Recording Industry Association of America (RIAA). She enjoyed significant commercial success with her subsequent Platinum and Gold-certified albums released through Hollywood Records; Hilary Duff (2004), Most Wanted (2005), and Dignity (2007). Following a hiatus from music, Duff signed with RCA Records for her fifth album, Breathe In. Breathe Out. (2015). Duff has been hailed as an inspiration by subsequent Disney teen stars such as Miley Cyrus, Demi Lovato, and Selena Gomez, and has sold an estimated 15 million records worldwide.

In addition to music and acting, she has also co-authored a trilogy of novels, beginning with Elixir (2010), which became a New York Times best seller, and followed by the sequels Devoted (2011) and True (2013). Duff's success in the entertainment industry led her to venture into business with fashion lines of her own such as Stuff by Hilary Duff, Femme for DKNY, and the "Muse x Hilary Duff" collection, a collaborative effort with GlassesUSA which was credited to have boosted GlassesUSA's sales for its premium brands. She has also invested in a number of businesses ranging from cosmetics to children's products.