Mary Margaret Helgenberger (born November 16, 1958) is an Emmy- and Golden Globe-nominated American film and television actress. Helgenberger is known for her role as Catherine Willows in CSI: Crime Scene Investigation, and her role as KC Koloski in China Beach, for which she won an Emmy.

Marg Helgenberger is an established dramatic actress whose prominence has been steadily increasing. Her work has been noted on stage, film and TV. Most of her career has been spent in dramatic roles on television, but she has also had a noteworthy presence in feature films. Helgenberger earned a degree in drama at Northwestern University. A talent scout recruited her from there to work on the soap opera "Ryan's Hope" (1975) where she appeared over the course of the next four years. Throughout the 1990s Helgenberger took on numerous roles in made-for-TV movies and as a guest star on many TV series. In particular she appeared in many movies made specifically for the Lifetime cable network and also for Showtime. She won critical acclaim for In Sickness and in Health (1992) (TV), Thanks of a Grateful Nation (1998) (TV) and Perfect Murder, Perfect Town: JonBenét and the City of Boulder (2000) (TV). In TV series she won an Emmy for her portrayal of a hard-bitten prostitute who catered to Vietnam War soldiers, in the series "China Beach" (1988). She also was George Clooney's love interest in a multi-episode arc of the monumentally successful TV series "ER" (1994). In feature films, Helgenberger has appeared in Tootsie (1982), Steven Spielberg (I)'s Always (1989), Species (1995) and In Good Company (2004). Her greatest claim to fame on the silver screen may be when she played opposite Julia Roberts (I) as a chemical exposure victim in the popular movie Erin Brockovich (2000). Helgenberger is most known for her TV role as a crime scene investigator in "CSI: Crime Scene Investigation" (2000). She shared in CSI's 2005 Screen Actors Guild award for outstanding performance by an ensemble in a drama series. In her personal life, Helgenberger is the daughter of a cancer survivor and is very active in supporting research for breast cancer.