Alyson Lee Hannigan (born March 24, 1974) is an American actress. She currently plays Lily Aldrin in the CBS sitcom How I Met Your Mother, but is best known for her previous role as Willow Rosenberg on the television series Buffy the Vampire Slayer. She also played flautist Michelle Flaherty in the American Pie film series.

Hannigan was born in Washington, D.C., the daughter of Emilie Posner, a real estate agent, and Al Hannigan, a truck driver. Hannigan is Jewish on her mother's side, and also has Irish ancestry. Her parents divorced a year after her birth and she was raised mostly by her mother in Atlanta.

Although Hannigan had appeared in an industrial film for "Active Parenting" as a baby, as well as starred in a commercial for the Duncan Hines cookie mix in 1978, it wasn't until she moved to Los Angeles, California in 1985 that she formally began her acting career. Living with her mother and attending North Hollywood High School, she successfully auditioned for agents while visiting her father in Santa Barbara.

Hannigan's first major film role was in My Stepmother Is an Alien, a science fiction comedy released in 1988; one of her co-stars in the film was actor Seth Green, who would later join her in the regular cast of Buffy as her on-screen boyfriend. Then in 1989, her first regular role on a TV series came when she was cast in the short-lived ABC sitcom Free Spirit. Throughout most of the early 1990s, she appeared in commercials and supporting roles on television shows, including Picket Fences, Roseanne and Touched by an Angel.

In 1997, Hannigan was cast to play Willow Rosenberg, Buffy's best friend, on the television series Buffy the Vampire Slayer (replacing Riff Regan, who played Willow in the unaired 25-minute pilot). The show became a success, and Hannigan gained recognition, subsequently appearing in several notable films aimed at teenage audiences, including American Pie, American Pie 2, Boys and Girls, and American Wedding. By the time Buffy had ended in 2003, Hannigan was earning a $250,000 salary for each episode. She also had a guest spot on the Buffy spin-off, Angel, reprising her role of Willow in a few episodes (including most notably "Orpheus", during the fourth season of Angel and the seventh season of Buffy), but none after Buffy finished production.

In early 2004, Hannigan made her West End debut, starring in a stage adaptation of When Harry Met Sally... at the Theatre Royal Haymarket, opposite Luke Perry.

In 2005, Hannigan returned to starring in a regular television series, appearing in the hit comedy, How I Met Your Mother, as Lily, and also playing a recurring guest role on Veronica Mars as Trina Echolls.

In February 2006, Hannigan starred as Julia Jones in Date Movie, a parody on romantic comedies.