In September 2000, Kat Dennings began her acting career with a guest spot in an episode of HBO’s popular dramatic sitcom starring Sarah Jessica Parker, “Sex and the City.” She followed it up with a regular role as Sarah Stewart (2001-2002) on the WB short-lived series "Raising Dad."

The next years saw her in the made-for-television movies The Scream Team (2002), The Snobs (2003) and Sudbury (2004). She was also spotted as a guest in an episode of CBS drama series "Without a Trace," ABC sitcom "Less Than Perfect," CBS crime drama "CSI: Crime Scene Investigation" and its second spin-off, "CSI: NY." Additionally, she could be seen in writer-director Sean McNamara's musical drama film starring Hilary Duff, Raise Your Voice (2004).

From 2005 to 2006 Kat played the recurring role of Zoe Butler on popular NBC medical drama "ER." During that time, she was cast in writer-director David Jacobson's romantic western drama film Down in the Valley (starring Edward Norton and Evan Rachel Wood), which debuted at the Cannes Film Festival, and Judd Apatow's directorial debut, the romantic comedy The 40 Year Old Virgin (starring Steve Carell), in which she played Catherine Keener's teenage daughter. She also appeared in writer-director Hunter Richards' biographical romantic drama London, alongside Chris Evans and Jessica Biel, and John Whitesell's comedy Big Momma's House 2, the sequel to the 2000 original starring Martin Lawrence.

Kat has wrapped her upcoming film with director Jon Poll, Charlie Bartlett, in which she co-stars with Anton Yelchin, Tyler Hilton, Robert Downey Jr. and Hope Davis. The comedy film is set to be released on August 3, 2007.