If a show's really, really lucky, it'll last a number of seasons. Some shows are lucky to get one. Meanwhile, some of the new programming premiering at the beginning of a season flops so badly that it's canceled before even being allowed to complete a full run.
This leaves the network with an uncomfortable situation - how to fill out the rest of the season. Often, networks will throw up reruns of popular shows in the failed show's timeslot, or fill the void with Made For TV Movies or other special programming. The usual go-to guy for such a situation, however, is the Midseason Replacement.
The Midseason Replacement is typically a show network execs don't consider strong enough to premiere at the beginning of a season, but is held in reserve for a premiere later when time slots are needed to be filled after another show has gone belly-up. Since the midseason replacement is considered to be a weaker show than what the network was originally hoping for, expectations often aren't very high. Another reason a show might be a midseason replacement is because of production problems - The Simpsons is one such notable example, as is Star Trek: Voyager, which simply had to wait for the rest of the network to premiere. Also, due to the need to stretch out episodes if a show airs over the entire TV season (September through May), a show may premiere midseason simply so they can air all the episodes with few or any breaks (this is especially common for serialized programming or reality shows).
Midseason replacements premiere, as the trope name suggests, in mid-season, which is typically anywhere from late December to early May. Any date later than that, and it's considered a late season replacement, which can premiere from late May into June. Late season replacements are usually considered by networks to be very weak shows, may have only a very limited number of episodes ordered, and are considered to have almost no chance of actually being picked up. Cancellations before the show even premieres are not rare. A late season replacement may result from a network wishing to fill gaps in its summer programming, to provide an alternative to summer rerun schedules, or may have been originally slated as a potential midseason replacement but in which feared gaps in midseason programming never materialized.
Note that this trope only specifically applies to broadcast network programming in the United States; different countries have different conventions, and the practices of cable networks in the United States may vary just as greatly. For example, USA Network's Monk, generally considered to have been one of the strongest shows on cable, premiered new episodes in the middle summer months and winter months, in a unique (though common practice on USA Network) season split-up. Compare, Dump Months in the movie industry for a similar time of year when films are perceived to be of lower quality.
- Reality Shows very frequently have midseason or late season premieres as emergency replacements for programming that has failed unexpectedly. Networks have pretty much refined the quick formulation, production and execution of reality shows (often in a matter of weeks from conception to premiere at most) to a fine art at this point.
- Most prime-time Animated Series are intentionally midseason replacements, the reasoning being that they're more expensive and time-consuming to produce and the networks don't want to have it all go to waste if the show bombs. So the first season is intentionally made shorter as a sort of trial run.
- Castle aired only ten episodes in its first season in a replacement slot after Dancing with the Stars, but then managed to have a total of eight seasons.
- Grey's Anatomy
- Happy Days as well as its spinoff, Laverne & Shirley.
- The Invaders (1967), which premiered in January 1967, was successful enough that ABC renewed it that fall.
- It Takes a Thief (1968), which premiered in January 1968 and lasted for three seasons.
- While the first three seasons of Lost premiered at the start of the fall season, the last three seasons all premiered mid-season with shorter episode counts to wrap it up toward a conclusion.
- Airwolf ran for three seasons after a Super Bowl launch on CBS and for a fourth on USA.
- All in the Family
- Becker didn't initially make CBS's fall schedule, but it did end up premiering in November because The Brian Benben Show had done so badly in the ratings that it got yanked after just four episodes.
- The Incredible Hulk started in March '78 as a mid-season replacement, so it only had 10 episodes in it's first season. Season's 2-4 had between 18 and 23 episodes each; then the show was mid-season replaced itself, so it's final season only had 7 episodes.
- It's noteworthy that the network officially launched in primetime in April 1987,note and as a result several of its more notable early shows such as Married... with Children, 21 Jump Street, and The Tracey Ullman Show did as well.
- 24's first three seasons all started in September and ran until May, but complaints about a show in real time being stretched out over the entire season caused them change to a "non-stop season" format beginning with season 4, in which every week had at least one new episode; for obvious reasons, for the finale to happen in May, the show had to premiere midseason.
- American Dad! likewise followed ''Family Guy' for its premiere on the first week of May 2005.
- Family Guy - having been canceled and then Un-Cancelled is notable enough, but its "uncancellation premiere" almost qualifies as a late season replacement having originally premiered the first week of May 2005. In a rare case of tactful thinking, FOX executives were confident enough of the show's popularity to bring in huge ratings despite the late premiere date. In its original run, the first, seven-episode season premiered after Super Bowl XXXIII in 1999.
- The original run of Futurama on Fox started out with midseason premiere.
- Most seasons of Power Rangers ran from February to November, despite originally premiering in the fall. The reason is that the continually shrinking source footage was wearing thin and the third season had a truncated number of episodes, so they created a mini-arc called "Mighty Morphin' Alien Rangers" (where the main team's morphing powers were destroyed and they called in to an alien Ranger team to protect the Earth) which served as a bridge between Mighty Morphin' Power Rangers and Power Rangers Zeo (where new powers are forged for the main team and the alien Rangers return home).
- The Simpsons: It was meant to be a fall premiere, but several early episodes had to be re-animated.
- Titus was a mid-season show.
- The A-Team premiered in midseason in 1983 and ran successfully for five seasons before Cancellation, although it did have the advantage of going after the Super Bowl. It still has a Cult fanbase and spawned a Movie in 2010 (some 27 years after its premiere).
- Series/Emergency! started in January 1972 with a 2-hour pilot, and from there, ran for the rest of the 1971-72 season, and then five more full seasons from 1972-1977.
- Law & Order
- Night Court
- The Office was not only a midseason premiere, it almost didn't last to a second season.
- Parks and Recreation
- Quantum Leap.
- Sanford and Son
- Seinfeld. It was threatened with cancellation at the conclusion of its initial season, a sparse 5 episodes, but was given a second chance, and the rest is history.
- Star Trek: Voyager premiered in January, launching the UPN network. It had a shortened first season and did well enough to last seven seasons.
- Buffy the Vampire Slayer. Its first season was only twelve episodes. Sarah Michelle Gellar even commented that when she told people what she was working on, they said she would get something better next season.
- Star Trek: Deep Space Nine began its run in January. Like Voyager, it had a shortened first season and ran for seven seasons.