A television pilot is the first episode of an intended television series. It is usually longer than normal episodes (often twice the normal length) and is intended to get network programming executives, and later the public, interested in the series. However, pilots are rarely fair examples of what a "normal episode" of a series is like, since they usually set the general background and tell the origin story for the series (e.g., if the series is about two angry roomates, the pilot will probably show how they met).
While many pilots are shot, few make it to the screen, and even fewer go on to become full-fledged television series. Competition at the network level is intense, with advertising dollars and choice viewer demographics at stake.
The concept for a pilot is generally "pitched" to network executives by a producer or writer. If interested, the network will fund the writing of a script. This may happen 50 times in a particular year. At this point various stakeholders at the network propose changes, and rewrites occur to satisfy those demands. If a project is unable to meet these changes, it will often be shelved or enter "development hell", a period of perpetual rewrites and recasting that lasts until the pilot is deemed completed or the producers give up on the project.
If the script for a pilot has satisfied the stakeholders at the network and is sufficiently exciting, then the production of the pilot itself can begin. On average, about 10% of the scripts commissioned by Hollywood networks actually get to the production stage.
Pilots are expensive to produce. Before a network commits to funding an entire pilot episode, it often requests a pilot presentation, a one-day shoot that, when edited together, gives a general idea of the look and feel of the proposed show. Some pilots can be just a few minutes long (ex: 10 minutes or less); however, such pilots will not be shown on the air unless more material is subsequently padded into them to make them at least twenty-two minutes in length, the actual duration of a nominally "thirty minute" program. Occasionally, more than one pilot is commissioned for a particular proposed television series to evaluate what the show would be like with modifications. Star Trek is the most famous example of this situation.
Pilots usually run as the first episode of the series, unless the series ended up being so different from the pilot that it wouldn't make sense (in this case the pilot, or portions of it, is reshot or rewritten to fit the rest of the series).
An example of change between the making of a pilot the making of a series is To Tell The Truth in 1956. The original title of the pilot was Nothing But The Truth and the show was hosted by Mike Wallace. The program host was changed to Bud Collyer, and the title changed to To Tell The Truth.
A backdoor pilot is a television movie or other TV special event that being used as a trial balloon—if audiences respond and ratings or good, the studio or network may order subsequent episodes from the creators. Again, Star Trek provides a famous example with the episode, "Assignment: Earth" where the crew of the Enterprise encounters Gary Seven, a character with his own agenda with the planet in a story that was intended to introduce the character and the premise of his adventures in a proposed series of his own.
At one time many pilots not selected for production as full series were aired as parts of anthology summer replacement programs, especially for popular series; these shows were regarded as potentially holding more of the audience than repeat showings of old episodes and this usage allowed the recoupment of at least a small part of the large development cost. Also, in a very few, rare instances, public interest could result in network executives taking a second look at the decision not to go forward with the series. This practice is now very seldom seen; pilots are more apt to be screened to focus groups than a mass audience; few pilots not selected to go into production as a series are ever aired in any form.