Born in Toronto, Polley was raised by parents that were both involved in show business; British-born father Michael and brother Mark were actors, while mother Diane, sister Joanna, and half-brother John Buchan were casting directors. Polley's interest in acting was sparked at age four after seeing her sibling's work, and despite her parents' protests, landed her first role on the Canadian crime series "Night Heat" (CTV/CBS, 1985-89) in 1985. That same year, Polley had a small but memorable role as a young girl whose family is too poor to buy presents in the lighthearted Disney fantasy "One Magic Christmas" (1985). The modest success of that film led to more television work and features in Canada and the United States, most of which, including "The Big Town" (1987) with Matt Dillon, were largely forgettable. In 1988, Polley starred as children's novelist Beverly Cleary's precocious heroine Ramona Quimby in the PBS series "Ramona" (1988), which, despite running for only 10 episodes, attracted a devoted following.That same year, Polley landed a key role in Terry Gilliam's epic fantasy "The Adventures of Baron Munchausen" (1988). Though visually impressive, the film ran perilously over budget and schedule, and Polley herself recalled being placed in harm's way throughout the production. The experience was so traumatic, that Polley swore to never appear in another big-budget Hollywood movie, prompting her in 2006 to write an email to Gilliam - which was printed in Toronto newspapers - that asked him to show more concern for the welfare of a young Canadian actress whom he had cast in his film "Tideland" (2006). After the "Munchausen" ordeal, Polley returned to Canada and found consistent work on television. She earned a Gemini Award for her performance in the Disney Channel feature "Lantern Hill" (1989), which was based on a book by popular Canadian young adult author, Lucy Maud Montgomery. Polley's connection to Montgomery's work continued with the Canadian/U.S.-produced TV series, "Road to Avonlea" (Disney Channel, 1990-1996). As an 11-year-old heiress who is sent to live with her mother's family on Prince Edward Island at the turn of the 20th century, Polley was top-billed on the period drama, and both she and the series received overwhelming acclaim by its primary audience of preteen girls.However, a pair of tragic events in Polley's personal life helped to turn her away from the warm, apple-cheeked world of "Avonlea" and mainstream entertainment on the whole. First, Polley's mother passed away two days after her 11th birthday; later that same year, she underwent painful treatment for a case of scoliosis - which was eventually corrected with surgery when she was 15 - a malady that forced her to drop out of a stage production of "Alice Through the Looking Glass." Both situations opened Polley's eyes to a world beyond acting. She instead began a growing interest in politics and culture, and this newfound curiosity did not sit well with her employers. After refusing to remove a necklace with a peace symbol at an event, she clashed with Disney, and left "Avonlea" as a regular in 1994. After "Avonlea," Polley pursued political activism with a vengeance - to the point that a police officer knocked out two of her teeth during a rally in 1995 - only dabbling in acting. But a small role in Canadian art-house filmmaker Atom Egoyan's "Exotica" (1994) led to a film that would reinvigorate her career and recast her as a mature leading lady with limitless potential and talent. Egoyan's "The Sweet Hereafter" (1997) told the story of a fatal school bus crash and the effect a court case has on its survivors and the families of the deceased. Polley riveted critics and audiences as a wheelchair-bound survivor whose horrific family life has a dramatic impact on her testimony. The film also featured four songs performed by Polley and its composer, Mychael Danna. For her efforts, Polley and her co-stars won the National Board of Review's award for Best Acting Ensemble.Meanwhile, Polley was winning more accolades for her performance as a goth-styled teen on the much-loved but short-lived Canadian TV series, "Straight Up" (CBC, 1996-98). Polley took home her second Gemini for her work on the show in 1998. Its early demise opened the door for the actress to make more features. From 1998 on, she appeared in a string of critically acclaimed independent features in her native country and the United States, including David Cronenberg's "eXistenZ" (1999), Doug Liman's sleeper hit "Go" (1999), and Audrey Wells' bittersweet romance "Guinevere" (1999), the latter about the relationship between a middle-aged photographer (Stephen Rea) and a younger woman (Polley). Reportedly both actors imitated their onscreen affair in real life for a brief period of time.The attention garnered by these films was not to Polley's liking, though; she famously vomited after being barraged by photographers at the premiere of "The Sweet Hereafter" and turned her back on bigger fame by quitting Cameron Crowe's "Almost Famous" (2000) after three months of rehearsal. Polley, who was cast as super groupie Penny Lane, returned to Canada and contemplated her future for several months, before exorcising her creative demons with her first effort as a writer-director: a short film called "The Best Day of My Life" (1999). Pleased with the experience, she made several more shorts, including 2001's "I Shout Love." She also began a relationship with film editor David Wharnsby, whom she married in 2003. Meanwhile, Polley's acting career continued to yield impressive results. She remained rooted in independent features from 2000 on, working with such noteworthy directors as Michael Winterbottom on the period drama "The Claim" (2000) and Hal Hartley in "No Such Thing" (2001), a curious updating of "Beauty and the Beast." Polley received some of the best reviews of her career as a low-income mother dying of cancer in the heartrending "My Life Without Me" (2003) for director Isabel Coixet, with whom she reunited in 2005 to play a deaf woman who cares for a severely burned oil rig worker (Tim Robbins) in "The Secret Life of Words."Polley made her most commercial film to date in 2004, starring in the horror hit "Dawn of the Dead" but in typical fashion, she returned almost immediately to the comforts of Canada and the independent world. In 2006, she appeared in five episodes of the much-loved Canadian TV comedy "Slings and Arrows" (TMN, 2003-06), which also starred her father, Michael. That same year, she worked with her husband on her feature directorial debut, "Away from Her" (2006), a difficult and moving film based on a short story by Alice Munro. Polley skillfully guided her exceptional cast, which included Julie Christie, Olympia Dukakis, Michael Murphy, and Canadian actor Gordon Pinsent, through a story about a man who struggles to care for his Alzheimer's-afflicted wife, even after she falls for another man at her nursing home. The picture generated largely positive reviews and brought awards from several international film festivals, as well as shedding a lot of light on Polley as more than just an accomplished actress. Polley was also bestowed with her first Academy Award nomination for Best Adapted Screenplay.Two years later, Polley portrayed Nabby Adams, eldest daughter of the founding father and future U.S. president (Paul Giamatti) in the acclaimed biographical miniseries "John Adams" (HBO, 2008). The actress earned high marks for her performance, which included a particularly harrowing account of Nabby's battle with breast cancer, including a reenactment of the crude mastectomy procedure she underwent in an ultimately failed attempt to fight off the disease. More feature film work included a co-starring role alongside Adrien Brody as a scientist whose gene-splicing research leads to an unexpected and ultimately deadly breakthrough in the science-gone-wrong horror tale "Splice" (2009). The year also included a little-seen turn opposite Jared Leto in the time-jumping romantic fantasy "Mr. Nobody" (2009). Written and directed by Belgian filmmaker Jaco Van Dormael, the philosophical fairy tale eventually achieved a modicum of cult status in the years that followed its limited release. Having divorced from Wharnsby in 2008, Polley married for a second time to legal Ph.D. student David Sandomierski, with whom she had a daughter, Eve, the following year.For her sophomore feature film effort, Polley teamed with stars Michelle Williams and Seth Rogen to write, produce, and direct "Take This Waltz" (2011). A romantic drama that eschewed tidy resolution, it told the story of a young Toronto couple (Williams and Rogen) whose seemingly comfortable marriage is tested by the wife's chance encounter with a charismatic neighbor (Luke Kirby). Polley next turned the camera on herself when she wrote and directed her first feature-length documentary, "Stories We Tell" (2012). Using family home movies, interviews, and dramatic reenactments, Polley conducted an investigation into her own family history, eventually leading to the revelation that her birth was the result of an extramarital affair that her mother had with producer Harry Gulkin, her actual biological father.