Dr. Gilbert "Gil" Grissom is the CSI unit night shift supervisor and a highly respected forensic entomologist with a degree in biology from University of California, Los Angeles. He is proficient in American Sign Language (ASL) because his mother (Phyllis Frelich) is deaf. He became a CSI in about 1985 and became supervisor for the Las Vegas CSI unit night shift in season one, episode two. Grissom is regarded as a well-educated methodical scientist but somewhat unusual in his approach toward his work and his social life, as well as a bit of a quirky introvert. In the series, some of his comments and actions can be seen to dumbfound his co-workers and superiors. He is portrayed as being a father figure to his team. It is revealed in the episode, Way To Go, that he has been in a relationship with fellow CSI, Sara Sidle. He proposes to her in The Case of the Cross-Dressing Carp.

In season nine, Grissom announced his retirement and in his final scene as a series regular in One to Go, he is shown meeting his fiancée, Sara Sidle, in the rain forest of Costa Rica. It is revealed in Family Affair that Grissom and Sara are now married.

The Grissom character is loosely based on real life criminalist Daniel Holstein. (Actor William Petersen was originally reported to have renewed his contract for the entire season nine, but the Associated Press reported that Petersen was leaving the show as a regular in episode 10 to pursue more stage-acting opportunities. He will return for guest spots during the show's run, as needed.)