After gaining fame for her starring role on the television show Lizzie McGuire, she has gone on to have a film career, with roles in high profile releases such as The Lizzie McGuire Movie, Cheaper by the Dozen, Cheaper by the Dozen 2 and A Cinderella Story. Duff has also expanded her repertoire into pop music, with three albums: 2003's Metamorphosis, 2004's Hilary Duff and 2005's Most Wanted.
Hilary Duff was born in Houston, Texas, the second child of Robert Erhard Duff, owner of a chain of convenience stores, and Susan Colleen Cobb, a homemaker. Duff's middle name of "Erhard" was the maiden name of her paternal grandmother, Mary Erhard, who was of part German descent. After Duff's mother encouraged her to take an acting class alongside her older sister, Haylie, both girls won parts in various local theatre productions. At the age of six, the Duff sisters participated in the ballet The Nutcracker Suite with Columbus Ballet Met in San Antonio. The siblings became more enthusiastic about the idea of acting professionally, and eventually relocated to California with their mother. Robert Duff stayed at the family home in Houston to maintain their business. After several years of auditions and meetings, the Duff sisters were cast in several television commercials and launched their careers. 
Most of Duff's first few acting roles were small, starting off with an uncredited appearance in Hallmark Entertainment's western miniseries True Women (1997). She also served as an extra, again uncredited, in writer/director Willard Carroll's ensemble comedy drama Playing by Heart (1998). Her first major part was as the star of the 1998 film Casper Meets Wendy, playing the young witch, Wendy, who encounters the animated character Casper. Like Casper: A Spirited Beginning (1997), the second sequel to the successful Casper (1995), the film was released direct-to-video with generally unenthusiastic reviews.
Duff later appeared in a supporting role in the television movie The Soul Collector (1999), which was based on a Kathleen Kane novel, and starred Bruce Greenwood as an angel who helps out a female farmer (Melissa Gilbert) whose husband has recently died. Duff ended up winning a Young Artist Award for Best Performance in a TV Movie or Pilot (Supporting Young Actress).
Duff's first serious shot at fame came when she was cast as one of the children in the pilot episode of the NBC sitcom Daddio (2000). Actor Michael Chiklis, co-star of Daddio, "After working with her the first day, I remember saying to my wife, 'This young girl is going to be a movie star'. She was completely at ease with herself and comfortable in her own skin". Before the show had even aired, Duff was dropped from its cast lineup and became reluctant to continue her acting career. Her manager and mother spurred her on, and she successfully auditioned for the family comedy show Lizzie McGuire a week later.
Lizzie McGuire, which first aired on the Disney Channel in January 2001, was a ratings hit, drawing in 2.3 million viewers per episode, and became the career breakthrough Duff had been waiting for. Her participation in the show led to her becoming highly popular among children between the ages of seven and fourteen, with critic Richard Huff of the New York Daily News calling her "a 2002 version of Annette Funicello". After Duff fulfilled her entire sixty-five episode contract, as well as participated in a film spin-off entitled The Lizzie McGuire Movie (2003), Disney toyed with the idea of continuing the franchise in further films and a prime-time television series to be broadcast on ABC, but Duff refused the proposal.
Duff's second role in a theatrical motion picture was in Human Nature (2002), an independent film first shown at the Cannes and Sundance film festivals. Written by Charlie Kaufman and directed by Michel Gondry, the film follows a female naturalist, played by Patricia Arquette. Duff played the younger version of Arquette's character.
Duff subsequently starred in the Disney Channel television film Cadet Kelly (2002), opposite Christy Carlson Romano and Gary Cole, which became the network's most watched program in its nineteen-year history. Her first major role in a feature film was in Agent Cody Banks with Frankie Muniz in 2003. The film was successful enough to spawn a sequel, in which Duff did not participate. After the commercially successful The Lizzie McGuire Movie was released in May of 2003, Duff played one of the twelve children of Steve Martin and Bonnie Hunt in the family film Cheaper by the Dozen, which remains her highest grossing film. She reprised her role in the sequel, Cheaper by the Dozen 2 (2005), which failed to repeat the financial success of the original film and was panned by critics.
In 2004, Duff starred in the romantic comedy A Cinderella Story, a twenty-first century update of Charles Perrault's fairy tale Cinderella. The film became a moderate box office hit, and some critics were impressed by Duff's performance, as well as her chemistry with co-star Chad Michael Murray. The movie earned a total of $66,068,046 worldwide and was a commercial success. Later that year, she starred in the film Raise Your Voice. While some critics praised Duff for appearing in a more dramatic role than she had previously been seen in, the film was heavily panned, with the Las Vegas Weekly writing: "Effortlessly combining Duff's bad acting and bad singing with bad writing and bad direction, Raise Your Voice is an insulting waste of time that begs to be silenced". Reviews were, by and large, also negative to Duff's vocals (several critics have pointed out what appears to be her digitally enhanced voice ) and indifferent towards her acting performance. Duff received a Razzie award nomination for "Worst Actress" (in addition to her work in A Cinderella Story). The film also received a muted reception at the box office, where it became Duff's least commercially successful film to date.
In Duff's next film, The Perfect Man (2005), she played the eldest daughter of a divorced woman (Heather Locklear) who moves to New York City as she desperately searches for a man to settle down with. Reviews were uniformly negative, and the film was a box office failure. That year, Duff was again nominated for a Razzie Award, for both The Perfect Man and Cheaper by the Dozen 2.
Her next film is the satirical comedy Material Girls. The Martha Coolidge-directed film, co-produced by Madonna's independent film production company Maverick Entertainment, stars Duff and her real-life sister Haylie Duff as wealthy siblings who must fight to reclaim their fortune following a scandal. It will be released in the U.S. on August 18, 2006. The Duff sisters are also due to lend their vocal talents to the computer animated comedy Foodfight!, to be distributed by Lions Gate Films in 2007. The film's director, Larry Kasanoff, said that he is absolutely thrilled to have the Duff sisters as part of the cast.
After seeing her sister perform in a band, Duff decided that she wanted to become a singer as well. She recorded the song "I Can't Wait" for the original television soundtrack for Lizzie McGuire in August 2002, and "The Tiki Tiki Tiki Room" for the first Disneymania compilation album released the following month. Her first album was Santa Claus Lane (2002), a collection of Christmas songs which included duets with Lil' Romeo, Christina Milian, and her sister, Haylie. It peaked well outside of the top 100 on the U.S. Billboard 200 album chart, and eventually achieved gold certification. The album's title track was included on the soundtrack to The Santa Clause 2. Duff sang several tracks for the soundtrack to The Lizzie McGuire Movie, including "Why Not". Released as a single, "Why Not" became a modest top twenty hit in Australia, New Zealand and the Netherlands.
Duff's first non-holiday album, Metamorphosis (2003), included contributions by songwriter-producers such as The Matrix and was #1 on U.S. and Canadian charts. It became one of the biggest selling albums of the year in the U.S. and has since gone to sell over 3.7 million copies. The lead single, "So Yesterday", was a top ten hit in several countries, while "Come Clean" became Duff's first top forty U.S. hit and reached the top twenty elsewhere. The final single, "Little Voice", was not released in the U.S. and was a minor hit in Canada and Australia.
The second Disneymania disc was released in January 2004, and contained a duet with her sister, Haylie, "The Siamese Cat Song". Another song, "Circle of Life", featured Duff and other Disney Channel Stars. Duff and her sister also recorded a cover of The Go-Gos' "Our Lips Are Sealed" for the soundtrack to A Cinderella Story.
Duff has stated that her second album, the self-titled Hilary Duff, had an edgier, rock feel to it. It was released on her seventeenth birthday (in September 2004) and debuted at number two in the U.S. and number one in Canada. The album has sold over 1.3 million copies in the U.S. to date, but the songs "Fly" and "Someone's Watching over Me" failed to achieve substantial success apart from reaching the Australian top forty.
Duff's third album, Most Wanted (2005), consisted of a selection of her favourite tracks from her previous two albums, alongside new songs inspired by rock musicians such as The Killers and Muse. Duff had more creative control over the album compared to her previous releases, co-writing and co-producing all of the new material with boyfriend Joel Madden. The lead single, "Wake Up", became Duff's highest debut on the Billboard Hot 100 and her highest peaking single in the U.S. The second single, "Beat of My Heart", was far less successful. The album itself debuted at number one on the Billboard 200 and became her third number one debut in Canada. By March 2006, it had sold 1.3 million copies in the U.S.
Duff has recorded three new songs for the soundtrack of her upcoming film, Material Girls, including a cover version of Madonna's "Material Girl". The track was produced by Timbaland. The film's soundtrack is going to be released on August 22, 2006.
There has been speculation regarding whether or not Duff performs live. She has said in an interview that she and her band are all live, but she was alleged to have lip synced a performance at MTV's New Year's Bash 2004.
Duff began dating singer Aaron Carter in 2000. The relationship lasted a year and a half, after which Carter left Duff for actress Lindsay Lohan, before reuniting with Duff. Carter further claims he cheated on Duff with her best friend, and has said that Duff "got her heart broken" and that he is "sorry" for his actions. Duff and Lindsay Lohan were later reported to have been involved in a "feud" with each other, over their relationship with Carter. As of 2006, the two have reportedly still not reconciled, and Duff has stated "Sometimes I feel like I really hate her [Lohan], which is pretty extreme for me".
Duff is currently dating Good Charlotte singer Joel Madden. The two announced their relationship in June of 2005, after a long period of tabloid speculation. In June of 2006, Duff announced that she is still a virgin. "Virginity is definitely something I like about myself. It doesn't mean I haven't thought about sex, because everyone I know has had it and you want to fit in."
Duff is involved with several charities, is an animal rights enthusiast, a member of "Kids with a Cause", and has donated US$250,000 to help the victims of Hurricane Katrina. She launched a clothing line, "Stuff by Duff", on March 12, 2004, with clothes distributed through Target in the United States, Kmart in Australia and Zellers in Canada. Playmates Toys also released a celebrity doll of her that year. 
In August 2005, Duff said she received veneers because she chipped off one of her front teeth on a microphone during a concert. In late 2005, Duff took a month off from work as a belated eighteenth birthday present. She said that she wanted to take a short break after releasing Most Wanted and writing three new songs for it, shooting three films, and embarking on her U.S. "Still Most Wanted Tour". By 2005, Duff also appeared to have lost weight, leading the media to speculate that she had developed an eating disorder, although Duff has denied this claim. Duff was interviewed on the Australian current affair show Today Tonight and stated that she lost weight by living a more healthy lifestyle.
In June of 2006, she made headlines by calling Madden's home town of Waldorf, Maryland "ghetto," site unseen.  They are scheduled to visit the town together in the summer of 2006.