Kat Dennings, born Katherine Litwack, is an American actress. She was born and raised in the Bryn Mawr area near Philadelphia, Pennsylvania. Her mother, Ellie Litwack, is a poet and speech therapist, her father, a molecular pharmacologist. Dennings is the youngest of five children. Her family is Jewish. She adopted the name Dennings as her professional name when she was younger. Kat was home schooled; her only enrollment at a traditional school was for a half-day at Friends' Central School. She moved with her family to Los Angeles, California around 2002 so that she could act full time. Her parents initially considered her idea of pursuing an acting career "the worst idea ever," Dennings told Interview magazine in 2007.

She began her career as a child in commercials. Dennings appeared on the HBO cable television series Sex and the City in 2000, where she played a 13-year-old girl who hires the character Samantha to handle publicity for her bat mitzvah in the episode "Hot Child in the City". Dennings was then cast as Bob Saget's daughter on the short-lived TV series, Raising Dad (2001– 2002). Since then, she has appeared in a number of television shows and movies, including the Hilary Dufffilm, Raise Your Voice (2004), and The 40-Year-Old Virgin (2005), playing Catherine Keener's daughter. Dennings also appeared as the character "Zoe" on several episodes of ER in 2005 and 2006, and was a guest star on CSI:NY, playing Sarah Endecott in the 2005 episode "Manhattan Manhunt". Dennings also appeared in the film Big Momma's House 2 (2006), starring Martin Lawrence.

In 2008, Dennings starred in Charlie Bartlett, the story of a wealthy teenager, played by Anton Yelchin who acts as a psychiatrist for his new public high school. Dennings played Susan Gardner, Bartlett's love interest, and the daughter of the school's principal, played by Robert Downey, Jr.. She also appeared in The House Bunny, playing Mona, a pierced, misfit sorority girl. Dennings stars with Michael Cera of Superbad and Juno, in the film Nick and Norah's Infinite Playlist (2008), a teen romance. She plays Norah Silverberg, the daughter of a famous record producer.

In 2009, Dennings will appear in Shorts, a dark children's film directed by Robert Rodriguez, and The Dream of the Romans, a story about a celebrity author whose manifestos become a sort of new Bible. Dennings hopes to make End Zone, a film based on the novel of the same name by Don DeLillo. She will also appear in the superhero film Defendor, starring Woody Harrelson and Sandra Oh where she will play a crack-addicted prostitute.