William Louis Petersen is an American actor. He is best known for his role as Gil Grissom in the CBS drama thriller series CSI: Crime Scene Investigation (2000–2015), for which he won a Screen Actors Guild Award and was nominated for a Golden Globe Award; he was further nominated for three Primetime Emmy Awards as a producer of the show. He reprised his role as Gil Grissom in the sequel CSI: Vegas, which premiered on October 6, 2021.

Gilbert Arthur Grissom, Ph.D. is a fictional character portrayed by William Petersen on the CBS crime drama CSI: Crime Scene Investigation and its sequel, CSI: Vegas. Grissom is a forensic entomologist and for the series' first nine seasons, a CSI Level III Supervisor employed by the Las Vegas Police Department. He appeared in 193 episodes of the original series, where he was succeeded by Raymond Langston and later D.B. Russell. Grissom returned in the sequel series, CSI: Vegas, in 2021.