The Prime Directive: According to Star Trek canon, it's the guiding principle of the United Federation of Planets and prohibits Starfleet personnel from interfering with the natural internal development of alien civilizations. First introduced about two-thirds of the way through the first season of the original series, the Prime Directive has cropped up in four of the five Trek series to date, with varying degrees of importance and success.
The problem with the Prime Directive is that while it prevents Starfleet or Federation personnel from interfering with other societies -- or worse, imposing its own values on them -- it also acts as a storytelling inhibitor. What is Captain Kirk supposed to do when he beams down to a planet where the local version of the Third Reich is about to launch its own Final Solution on another race of people -- just stand there? The way Star Trek was set up, the show almost forced Kirk repeatedly into situations where he had to violate the Directive, and he did so. Otherwise we'd be staring at a blank screen for most of that hour.
Here are 11 times that Kirk intervened in a society on another planet and violated the Prime Directive, along with one time that he played by the rules. Was he right or wrong?