Gillian Leigh Anderson, OBE (/ˈdʒɪliən/ JIL-ee-ən; born August 9, 1968) is an American actress. Her credits include the roles of FBI Special Agent Dana Scully in the series The X-Files, ill-fated socialite Lily Bart in Terence Davies's film The House of Mirth (2000), DSU Stella Gibson in the BBC/RTÉ crime drama television series The Fall, sex therapist Jean Milburn in the Netflix comedy-drama Sex Education, and British Prime Minister Margaret Thatcher in the fourth season of Netflix drama series The Crown. Among other honors, she has won two Primetime Emmy Awards, two Golden Globe Awards, and four Screen Actors Guild Awards. She has resided in London since 2002, after earlier years divided between the United Kingdom and the United States.

Born in Chicago, Anderson grew up in London, UK and Grand Rapids, Michigan. She graduated from The Theatre School at DePaul University in Chicago, then moved to New York City to further her acting career. After beginning her career on stage, she achieved international recognition for her role as FBI Special Agent Dana Scully on the American sci-fi drama series The X-Files. Her film work includes the dramas The Mighty Celt (2005), The Last King of Scotland (2006), Shadow Dancer (2012), Viceroy's House (2017) and two X-Files films: The X-Files: Fight the Future (1998) and The X-Files: I Want to Believe (2008). Other notable television credits include: Lady Dedlock in Bleak House (2005), Wallis Simpson in Any Human Heart (2010), Miss Havisham in Great Expectations (2011), Dr. Bedelia Du Maurier on Hannibal (2013–2015), and Media on American Gods (2017).

Aside from film and television, Anderson has taken to the stage and received both awards and critical acclaim. Her stage work includes Absent Friends (1991), for which she won a Theatre World Award for Best Newcomer; A Doll's House (2009), for which she was nominated for a Laurence Olivier Award, and a portrayal of Blanche DuBois in A Streetcar Named Desire (2014, 2016), winning the Evening Standard Theatre Award for Best Actress and receiving a second Laurence Olivier Award nomination for Best Actress. In 2019, she portrayed Margo Channing in the stage production of All About Eve for which she received her third Laurence Olivier Award nomination.

Anderson has supported numerous charities and humanitarian organizations. She is an honorary spokesperson for the Neurofibromatosis Network and a co-founder of South African Youth Education for Sustainability (SAYes). She was appointed an honorary Officer of the Most Excellent Order of the British Empire (OBE) in 2016 for her services to drama.

Anderson was born in Chicago, Illinois, the daughter of Rosemary "Posie" Alyce (née Lane), a computer analyst, and later vice president of Neurofibromatosis Inc., the NF support group of West Michigan, and Homer Edward "Ed" Anderson III, who owned a film post-production company. She is of English, German, and Irish ancestry. Soon after her birth, her parents moved to Puerto Rico for 15 months, then to London. The family relocated so that her father could attend the London Film School. With her parents, she lived in north London's Crouch End and Haringey. She was a pupil of Coleridge Primary School. When Anderson was 11 years old, her family returned to the United States, settling in Grand Rapids, Michigan. They continued to keep a flat in London and spent their summers there. Anderson later said that she had always intended to return to England. In Grand Rapids, she attended Fountain Elementary and City High-Middle School, a program for gifted students with a strong emphasis on the humanities.

We were in a small Republican town. There were only six punks there. We were weird. It's not like London.

Following the move to Grand Rapids, Anderson went through a rebellious stage; taking drugs, dating a much older boyfriend, and cultivating a punk appearance (dyeing her hair various colors, shaving the sides of her head, sporting a nose piercing and an all-black wardrobe). She was put in therapy at the age of 14. Anderson listened to bands such as Dead Kennedys and Skinny Puppy. She was voted by her classmates as "class clown", "most bizarre girl" and "most likely to be arrested". She was arrested on graduation night for breaking and entering into her high school in an attempt to glue the locks of the doors. She later managed to reduce the charges to trespassing.

At an early age, Anderson was interested in marine biology, but after becoming interested in theatre during her teenage years, she began acting in high school productions during her first year and later in community theatre. She also served as a student intern at the Grand Rapids Civic Theatre & School of Theatre Arts. After graduating from high school in 1986, she attended The Theatre School at DePaul University in Chicago, where she earned a Bachelor of Fine Arts in 1990. Anderson also participated in the National Theatre of Great Britain's summer program at Cornell University. To support herself financially during her student years, she worked at the Goose Island Brewpub in Chicago. After Anderson became famous, the brewery named one of their beers after her – a Belgian-style farmhouse ale called "Gillian".