Mary Marg Helgenberger (born November 16, 1958) is an American film and television actress known for her roles as Catherine Willows in the CBS drama CSI: Crime Scene Investigation from 2000 to 2012 also in 20 (TV movie finale), and as K. Koloski in the ABC drama China Beach, which earned her the 1990 Emmy Award for Outstanding Supporting Actress in a Drama Series.