William Petersen is an American actor and producer, best known as Dr. Gilbert 'Gil' Grissom on CBS' CSI. Other television credits include Colonel Emmett Darrow on WGN’s Manhattan and Captain Gideon Walker in the CBS miniseries, Return to the Lonesome Dove.

Film credits include CSI: Immortality, Seeking a Friend for the End of the World, Detachment, This Old Cub, Haven, The Contender, The Rat Pack, Gushy, Kiss the Sky, The Staircase, 12 Angry Men, Fear, Mulholland Falls, Cousins, Long Gone, Manhunter, To Live and Die in L.A., and Thief.

Theatre credits include his Broadway debut as Reverend T. Lawrence Shannon in The Night of the Iguana (Criterion Center Stage); and regionally, as Sterling in Slowgirl (Geffen Playhouse), Hamm in Endgame, John Plunkett in Dublin Carol, Eddie in Fool for Love, and Joe Conroy in Balm in Gilead (Steppenwolf Theatre); and his JEFF Award-winning performance as Ray in Blackbird (Victory Gardens Theater).