Allison Lee Hannigan (born March 24, 1974), known professionally as Alyson Hannigan, is an American actress and television presenter.

Hannigan began her film career with supporting roles in the comedy films Impure Thoughts (1986) and My Stepmother Is an Alien (1988), getting a Young Artist Award nomination for the latter. In 1999, she began starring in the American Pie film series as Michelle Flaherty, the films' primary love interest, appearing in every film in the series from 1999 to 2012. For her role in the series, she was nominated for three Teen Choice Awards and won a Young Hollywood Award. She went on to star in the parody film Date Movie (2006), the slasher film You Might Be the Killer (2018), and the superhero film Flora & Ulysses (2021).

Hannigan got her start in television starring in the short-lived sitcom Free Spirit from 1989 to 1990, for which she earned a Young Artist Award nomination. After several minor roles in television films and other series, she appeared in her breakout role as teenage witch Willow Rosenberg in the supernatural drama series Buffy the Vampire Slayer, appearing in every episode from 1997 to 2003. Her role on the show was critically acclaimed and won her the Saturn Award for Best Supporting Actress on Television and a Teen Choice Award. From 2005 to 2014, she starred as Lily Aldrin in the sitcom How I Met Your Mother, for which she won two People's Choice Awards. Hannigan hosted the television show Penn & Teller: Fool Us (2016–2023), and provided the voice of Claire Clancy in the Disney Junior animated series Fancy Nancy (2018–2022).