"Gunsmoke" aired from 1955 to 1975 and had 635 total episodes, making it the longest-running live-action drama ever aired during prime time as of 2015. It was based on a radio drama series that aired from 1952 to 1961.
The original radio program was conceived as a more mature alternative to Westerns like "The Lone Ranger" and "The Cisco Kid." The ongoing story mostly revolves around the adventures of Matt Dillon, a U.S. Marshal in Dodge City, Kansas, as he keeps the peace along with his deputies.
"Gunsmoke" had the largest viewership of any television program from 1957 to 1961. It was nearly canceled in 1967 after several years of lower ratings, but fan demand kept it on the air. The show instead moved to a new time slot and went back up into the top 10 of the Nielsen ratings. It did not have a special final episode, as the cast and crew were surprised by a sudden cancellation in 1975.
CBS would go on to make five made-for-television movies based on the series, with a number of series actors reprising their roles. The first of these, "Gunsmoke: Return to Dodge," aired in 1987. The last of the movies, "Gunsmoke: One Man's Justice," aired in 1994.