"Killer Within" is the fourth episode of the third season of the post-apocalyptic horror television series The Walking Dead. It originally aired on AMC in the United States on November 4, 2012, and was written by Sang Kyu Kim and directed by Guy Ferland. The episode marked Sarah Wayne Callies and IronE Singleton's final appearances as Lori Grimes and Theodore "T-Dog" Douglas and as regular actors on the show.