Most of the longest-running shows are fairly inexpensive to produce. Networks often favor the bottom line over critical praise, and a number of shows with high ratings and loyal audiences have been cancelled due to their high production budgets.

While the production budget of an average network TV show is $3.5 million per episode, long-running shows such as “South Park” cost an estimated $1.5 million per episode to produce. Some Nickelodeon cartoons cost less than $1 million per episode. Many of the shows with the longest lifespans are unscripted reality programs, which are far less expensive to produce than scripted television shows.

Other long-running television shows revolve around a charismatic host and a tried-and-true show format. Johnny Carson and Jay Leno were some of the most successful late-night talk show hosts in history, each hosting “The Tonight Show” for more than two decades. David Letterman hosted “The Late Show” for 22 years.

Many of the shows on this list practically invented new television genres. “60 Minutes,” for example, was the first news program to focus largely on investigative reporting. When “Saturday Night Live” first aired in 1975, it was one of the first shows to feature high-concept comedy sketches, political satire, and musical performances with a varying cast and a different host every week. The format has since been repeated in fewer successful iterations.

Many of the shows with the longest series runs are on the Big Three major broadcast networks: ABC, CBS, and NBC. Originally founded as radio broadcasting companies, ABC, CBS, and NBC each began commercial television operations in the 1940s, where they dominated and remained largely unchallenged for several decades. PBS was founded in 1970 and remains the largest nonprofit, government-funded broadcasting network. Of the 50 longest running shows of all time, 15 were first aired before the 1980s, when competitor Fox was founded and a number basic cable networks such as Nickelodeon and WTBS — a precursor to TBS — began to garner large shares of the television-viewing audience.

Of the 50 longest running television shows, 11 were originally broadcast on PBS, 10 on CBS, eight on NBC, two on ABC, and three on Fox. “The Original Amateur Hour,” the precursor to shows such as “American Idol” and “America’s Got Talent,” originally aired on Dumont, an early television broadcast network that lasted from 1946 to 1956. Many of the more recent programs among the 50 longest running TV shows, such as “South Park,” “The Challenge,” and “The Fairly OddParents,” air on basic cable networks Comedy Central, MTV, and Nickelodeon, respectively.

To determine the longest-running TV shows of all time, 24/7 Wall St. developed a list of primetime television shows using the Internet Movie Database and other sources. To be considered, a show must have aired in the primetime slot at one point during its original broadcast run. Anthology series, music television, and sports programs were not included. The length of a series run only consists of the time a show was actually on the air, not any time spent in cancellation or between networks. As a result, the length of a series run may be different than the range of years that a show has been in existence.