The original CSI: Crime Scene Investigation aired for 15 seasons.
The series premiered on CBS on October 6, 2000, and concluded on September 27, 2015. Its 15-season run followed forensic investigators in the Las Vegas Crime Lab as they examined evidence and reconstructed crimes. The original series produced 337 episodes.
CSI's final season was shorter than many earlier seasons, containing 18 episodes. The story concluded with “Immortality,” a feature-length, two-hour finale that brought back several important characters, including William Petersen's Gil Grissom and Marg Helgenberger's Catherine Willows.
The number can be confused with the broader franchise or with the show's spin-offs. CSI: Miami ran for 10 seasons, CSI: NY ran for 9, and CSI: Cyber ran for 2. Those totals are separate from the 15-season run of the original Las Vegas series.