Prime Directive - Temporal Prime Directive

The Temporal Prime Directive is intended to prevent a time traveler (from the past or future) from interfering in the natural development of a timeline. The TPD was formally created by the 29th Century, and was enforced through an agency of Star Fleet called the Temporal Integrity Commission, which monitored and restricted deviations from the natural flow of history. However, several Star Trek: Voyager episodes specifically make references to the Temporal Prime Directive that suggest that it applies in the 24th century.

The directive is regarded as "inviolable," and any Star Fleet officer responding to a question regarding their prior actions with words to the effect of "I cannot reply due to the Temporal Prime Directive" would not normally be subject to censure, as long as some form of temporal instability had been sensed, however slight the signs.

As 31st Century time traveler Daniels revealed to Captain Jonathan Archer in the Star Trek: Enterprise episode "Cold Front," as time travel technology became practical, the Temporal Accords were established sometime significantly prior to the 31st Century, in order to allow the use of time travel for the purposes of studying history, while prohibiting the use of it to alter history. Some factions rejected the Accords, leading to the Temporal Cold War that served as a recurring storyline during the first three seasons of that series.