Premiering on CBS in September 1955 and completing its network run September 1975, Gunsmoke is the longest running dramatic series in the history of television. Two of its stars, James Arness and Milburn Stone, remained all 20 seasons, with Amanda Blake a close second, departing after 19 years. 

The series started out as a half-hour show, and expanded to an hour in its seventh season. Prior to Gunsmoke (and The Life and Legend of Wyatt Earp which premiered the same week), western shows generally focused on fantasy characters such as the Lone Ranger and Hopalong Cassidy, holdovers from movie and radio serials. Gunsmoke was one of the earliest "adult westerns," centering around the exploits of Marshal Matt Dillon (James Arness) in the frontier town of Dodge City, Kansas in 1873. His kindly companion was Doc Adams (Milburn Stone), the town physician who spent many hours chugging beers at the Long Branch Saloon, owned and operated by the shapely Kitty Russell. 

Over the years there were several changes in the supporting cast, most notably the replacement of Matt's loyal deputy, Chester Goode (Dennis Weaver), with hillbilly deputy Festus Haggen (Ken Curtis). 

Gunsmoke started a longtime trend for TV westerns. At one point there were 30 of them on the air at the same time. But Gunsmoke outlasted the others and at the time of its cancellation in 1975, it was the only show of its kind still on the air.

Gunsmoke is currently running on the cable channel, Encore Westerns Channel. To find out what time it's playing in your area check out Encore Westerns Channel.