Mary Marg Helgenberger (born November 16, 1958) is an American actress. She is best known for her roles as Catherine Willows in the CBS drama CSI: Crime Scene Investigation (2000–2013), and as K.C. Koloski in the ABC drama China Beach (1988–1991), which earned her the 1990 Emmy Award for Outstanding Supporting Actress in a Drama Series. She has also appeared in the series Ryan's Hope (1982–1986) and the films Species (1995), Species II (1998), Erin Brockovich (2000), and Mr. Brooks (2007). She also starred in the series Intelligence alongside Meghan Ory and Josh Holloway.