William Louis Petersen (born February 21, 1953) is a Golden Globe and Emmy nominated American actor and producer, best known for playing Dr. Gilbert "Gil" Grissom on the hit CBS series CSI: Crime Scene Investigation. He played President John F Kennedy in 1998 TV film The Rat Pack.

Petersen, the youngest of six children, was born in Evanston, Illinois, to parents who worked in the furniture business. His father is Danish-American, and his mother is German-American. He graduated from Bishop Kelly High School in Boise, Idaho, in 1972. He was accepted to Idaho State University on a football scholarship. While at Idaho State, Petersen took an acting course which changed the direction of his life. He left school along with his wife, Joanne, in 1974 and followed a drama professor to Spain (Basque country) where he studied as a Shakespearean actor. Petersen was interested in Basque culture and he studied the Basque language, Euskera, and gave his daughter the Basque name Maite (meaning love). Petersen returned to Idaho intent on being an actor. Not wanting to work a non-acting job in Idaho, he returned to the Chicago area, living with relatives. He became active in the theater and earned his Actors' Equity card. He performed with the Steppenwolf Theatre Company and was a co-founder of the Remains Theater Ensemble which also included other prominent Chicago actors Gary Cole and Ted Levine.

He is usually credited without his middle initial (i.e. credited as "William Petersen" and not "William L. Petersen"). Because his role in Manhunter was so emotionally exhausting, he did everything he could to rid himself of Will Graham after finishing principal photography. He shaved off his beard, cut his hair and dyed it blonde. He also claims to have done this because, while rehearsing for a play in Chicago, his dialogue was always coming out Will Graham; he dyed his hair so he could look in the mirror and see a different person. In a move perhaps indicative of his career choices, Petersen declined a part in Oliver Stone's Platoon, as it would have kept him in the Philippines, away from his family. Instead, he worked on the 1987 HBO made-for-TV movie Long Gone.