Alyson Lee Hannigan (born March 24, 1974) is an American actress. She is best known for her roles as Michelle Flaherty in three American Pie films, Lily Aldrin on the CBS sitcom How I Met Your Mother and Willow Rosenberg on the television series Buffy the Vampire Slayer.

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.

Hannigan previously dated Ginger Fish of Marilyn Manson.