Born in Washington D.C. to Albert Hannigan and Emilie Posner, Hannigan grew up in Atlanta, GA with her real estate agent mother after her parents' divorce in 1972. The camera loved this auburn-haired actress from an early age. At only three months old, she was featured in an instructional film, "Active Parenting" (1974). Hannigan began to model as a child, posing for close to 100 print advertisements before making the transition into commercial acting. At the age of four, Hannigan landed her first commercial, promoting Duncan Hines cookie mix. She was later featured in a number of national TV commercials, including campaigns for McDonald's and Oreo cookies. Growing up in Atlanta, Hannigan attended the Northside School of the Performing Arts. With her burgeoning success in commercial acting, the 11-year-old Hannigan and her mother relocated to Los Angeles so that she could pursue her career more seriously.Hannigan's first breakthrough role came in 1988 when she was cast as Dan Aykroyd's suspicious daughter in the misbegotten comedy, "My Stepmother Is an Alien." She then landed a regular role on the ABC sitcom, "Free Spirit" (1989-90) and went on to land a number of other small television roles while still in school, including spots on "Roseanne" (ABC, 1988-1997), "The Torkelsons" (NBC, 1991-93), "Touched by an Angel" (CBS, 1994-2003) and "Picket Fences" (CBS, 1992-96). After graduating high school at North Hollywood High in 1992, Hannigan took some time off to attend college at California State University, where she majored in psychology. Returning to work in 1997, Hannigan landed her biggest break yet when she was cast as Willow Rosenberg, Buffy's (Sarah Michelle Gellar) quirky best friend on Joss Whedon's cult TV series, "Buffy the Vampire Slayer" (The WB, 1997-2001; UPN, 2001-03). With her newfound recognition, Hannigan started landing roles in a number of films geared towards a teenage audience, including "Dead Man on Campus" (1998), "Boys and Girls" (2000), and her biggest film to date - the unexpected teen sex comedy hit, "American Pie" (1999). Playing off her natural geekiness in the latter, Hannigan portrayed the sweetly perverted, flute-playing band geek Michelle Flaherty opposite love interest Jason Biggs. Hannigan uttered the franchise's most infamous line: "One time, at band camp," culminating in a hilariously X-rated explanation of what really did happen at band camp with that flute. The actress reprised her "Pie" role in the sequels "American Pie 2" (2001) and later, more prominently in the disappointing "American Wedding" (2003). Crossing over to the "Buffy" spin-off, "Angel" (The WB, 1999-2004), Hannigan reprised her role as Willow Rosenberg, appearing on the series in a number of episodes. In 2004, Hannigan made her stage debut opposite Luke Perry, starring in a stage adaptation of "When Harry Met Sally" at the Theatre Royal Haymarket in London's West End. Returning to series television in 2005, Hannigan guest-starred on "Veronica Mars" (UPN, 2004-07) and landed a regular spot on the CBS sitcom hit "How I Met Your Mother" (2005-14) portraying kindergarten teacher Lily Aldrin. With such a workload, Hannigan found time to return to the big screen in the 2006 send-up of cheesy romantic chick flicks, "Date Movie," a modest hit.Primarily occupied with her regular work on her ongoing hit "How I Met Your Mother," Hannigan made room in her schedule to guest star in a 2011 episode of "The Simpsons" (Fox, 1989-), as a potential love-interest for a decidedly indifferent Bart. Briefly back on the big screen after a five year absence, she popped up with a small supporting role in the romantic-comedy, "Love, Wedding, Marriage" (2011) alongside real-life husband and former "Buffy/Angel" co-star, Alexis Denisof. As "How I Met Your Mother" enjoyed continued success in its seventh season as one of network TV's most popular sitcoms, Hannigan and the old gang from East Great Falls returned once more to relive past infamies and endure fresh embarrassments in "American Reunion" (2012), the third sequel to the raunchy sex-comedy franchise which saw all the original cast back on board.