Alyson Hannigan (born Alyson Lee Hannigan on March 24, 1974) is an American actress who plays Lily Aldrin in the CBS sitcom How I Met Your Mother. She is also known for her previous roles as Willow Rosenberg on the television series Buffy the Vampire Slayer, flautist Michelle Flaherty in the American Pie film series and Julia Jones in Date Movie.