Dr. Gilbert "Gil" Grissom, (Ph.D.) is a fictional character portrayed by William Petersen on the American TV crime drama CSI: Crime Scene Investigation. Grissom was a forensic entomologist and the night-shift supervisor of the Clark County, Nevada CSI (forensics) team, investigating crimes in and around the city of Las Vegas. He was the show's protagonist from seasons 1 to 9, when Petersen left the show as a regular, and was replaced by Laurence Fishburne as Dr. Raymond Langston (M.D.).
CSI: Crime Scene Investigation producer and writer Carol Mendelsohn considers Grissom the center of the show, and in the first seven years of the show he appeared in every episode, with the exception of "Hollywood Brass", from season five, "Gum Drops" and "The Unusual Suspect" from season six, and "Sweet Jane" and "Redrum" from season seven. In early season 6 he became romantically involved with subordinate CSI Sara Sidle.
Grissom has received positive responses from critics, ranking number 82 on Bravo's list of Greatest Television Characters of All Time, along with Catherine Willows. Grissom's final episode as a regular drew over 23 million viewers.