Which actress portrayed Catherine Willows on CSI?

Marg Helgenberger portrayed Catherine Willows on CSI: Crime Scene Investigation.

Catherine is a Las Vegas crime-scene investigator and a specialist in blood-spatter analysis. The character begins the series as Gil Grissom's assistant supervisor and becomes one of the team's most experienced investigators. Her background is unusual for the laboratory: the series establishes that she was born and raised in Las Vegas and previously worked as a stripper before entering law enforcement.

Helgenberger was part of CSI's original principal cast and played Catherine throughout the show's main run. Catherine eventually succeeds Grissom as supervisor of the Las Vegas crime lab, although departmental politics and a scandal later affect her position. She leaves Las Vegas for the FBI, then returns during the series finale, “Immortality.”

The other options are also connected to CSI, which makes this question easy to confuse. Louise Lombard played Sofia Curtis, Liz Vassey played DNA technician Wendy Simms, and Jorja Fox played investigator Sara Sidle. Catherine's defining forensic specialty is blood-spatter analysis, while Helgenberger is the actor who brought her to the screen.