Hilary Erhard Duff (born September 28, 1987) is an American actress, singer, author and businesswoman. Her accolades include a World Music Award, seven Kids' Choice Awards, four Teen Choice Awards, and two Young Artist 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), A Cinderella Story (2004), and Cheaper by the Dozen 2 (2005). 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) which earned her award nominations, and produced and starred as Sophie Tompkins in the Emmy-winning Hulu sitcom How I Met Your Father (2022–2023).

In 2002, Duff entered music and her debut studio album, the Christmas-themed Santa Claus Lane, was released that year through Walt Disney Records. Duff later signed with Hollywood Records and achieved greater success with her second album, Metamorphosis (2003), which topped the Billboard 200 chart and earned a 4× Platinum certification from the Recording Industry Association of America (RIAA). She also enjoyed significant commercial success with her subsequent albums, Hilary Duff (2004) and Dignity (2007), as well as the 2005 compilation Most Wanted which all went to be certified platinum and gold. Following a decrease in musical activity, Duff signed with RCA Records for her fifth album, Breathe In. Breathe Out. (2015) before transferring to Atlantic Records in 2025. She has been hailed as an inspiration by subsequent teen Disney Channel stars such as Miley Cyrus and Selena Gomez, and has sold an estimated 15 million records worldwide. Billboard ranked her at number 73 on their "Top 100 Women Artists of the 21st Century Chart" list. She appeared on Forbes' annual Celebrity 100 list from 2004 to 2007.

In addition to music and acting, she has also co-authored a trilogy of novels, beginning with Elixir (2010), which topped The New York Times Best Seller list, 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 "Muse x Hilary Duff" with GlassesUSA. She is the Chief Brand Director of Below 60°, a line of home fragrance products. She has also invested in a number of businesses ranging from cosmetics to children's products.

Hilary Erhard Duff was born on September 28, 1987, in Houston, Texas. Her parents are Robert Erhard Duff, a partner in a chain of convenience stores, and Susan Colleen Duff (née Cobb), a homemaker turned film and music producer. Duff has one older sister named Haylie. She was raised between Houston and San Antonio, the locations of her father's convenience stores. Encouraged by their mother, both Hilary and her sister enrolled in acting, singing, and ballet classes. The siblings earned roles in local theater productions, and later participated in a touring BalletMet production of The Nutcracker in San Antonio. Increasingly interested in the pursuit of show business, the Duff sisters and their mother moved to California in 1993, while their father stayed in Houston to take care of his business. The sisters auditioned for several years and were cast in many television commercials. Due to her acting career, Duff was home-schooled from the age of eight. The pair also modeled for various clothing brands. Duff stated, "My sister and I really showed an interest in [performing] and dedication, and [our mother] was like, 'How can I tell my kids no?' It's the same as kids that are going into sports. Parents support them and push them."

During her initial acting years, Duff primarily played minor roles, such as her uncredited part in the Hallmark Entertainment western miniseries True Women (1997) and as an uncredited extra in the ensemble comedy-drama Playing by Heart (1998). The same year, Duff landed her first major role as Wendy in Casper Meets Wendy, based on the Harvey Comics characters. After appearing in the supporting role of Ellie in the television film The Soul Collector (1999), Duff received a Young Artist Award for Best Performance in a TV Movie or Pilot (Supporting Young Actress). In March 2000, Duff appeared as a sick child in the CBS medical drama series Chicago Hope. She was next cast as one of the children in the pilot episode of the NBC comedy series Daddio. Her Daddio co-star Michael Chiklis stated, "After working with her the first day, I remember saying to my wife, 'This young girl is gonna be a movie star.' She was completely at ease with herself and comfortable in her own skin." However, producers dropped Duff from the cast prior to the airing of the show.

A week after being dropped from Daddio, Duff landed the title role of a newly developed Disney Channel series, Lizzie McGuire. Lizzie McGuire premiered on January 12, 2001, and became a ratings hit, with an estimated 2.3 million viewers per episode. Duff quickly became labeled as a teen idol and household name, particularly within the show's pre-teen adolescent target demographic. Disney began marketing the series through soundtracks, books, dolls, toys, and video games based on Duff's character; the company reportedly made an estimated $100 million from the show's merchandise alone. The actress was featured in the Game Boy Advance video games that were created for her Lizzie McGuire character. Duff made her feature film debut in Human Nature (2001), in which she portrayed the younger version of Patricia Arquette's character. Duff also starred in the 2002 Disney Channel television film Cadet Kelly, for which she took a month of military training. The film became the network's most watched program in its then 19-year history.

Duff began making appearances on various soundtracks for the Disney channel, recording a cover of the Brooke McClymont song "I Can't Wait" for the soundtrack to Lizzie McGuire. She later recorded a cover of "The Tiki Tiki Tiki Room" for DisneyMania. Upon expressing interest in a music career, production on Duff's debut Christmas themed album began. Santa Claus Lane was released in October 2002, peaking in the lower portion of the Billboard 200 chart in the United States. The album was initially distributed through Walt Disney Records, with Buena Vista Records later re-releasing the project. It went on to receive a Gold certification from the Recording Industry Association of America (RIAA) for shipments exceeding 500,000 copies. The album was initially only released in North America, and received numerous re-releases in the following years. Duff later signed a recording contract with Andre Recke of Hollywood Records to release future recordings. Though her early music recordings garnered much airplay on Radio Disney, Recke and Buena Vista Records envisioned Duff appealing to a more mature audience.