Laws of Time

The Laws of Time were a set of guidelines and laws guiding what a time traveller may do. As opposed to physical Laws, which were observations of constants of behavior found in nature (ie The Laws of Motion or Gravitation), the Laws of Time were rules or guidelines put in place to prevent massive changes to the primary timeline by time travellers.

They traditionally prevented Gallifrey's 'present' from interacting with its subjective past or future. (NA: Lungbarrow).

It is unknown how those law were enforced, or what the exact punishment for breaking them was. It is possible the effects of breaking them were serious enough that they would themselves serve as a deterrent.

The Doctor has stated that he was "...Defender of the Laws of Time" in his seventh persona and eighth persona. (DW: Remembrance of the Daleks, EDA: Vampire Science)

The Laws of Time became weakened during the Second War in Heaven so that future events filtered back to influence interact with events prior to the war's outbreak. (EDA: Alien Bodies, The Taking of Planet 5)

Specific Laws of Time
One of the Laws of Time supposedly prevented Time Lords from meeting their previous selves, but this was broken several times in emergencies (DW: The Three Doctors, The Five Doctors, BFA: The Sirens of Time, IDW: Room with a Deja View) and at least twice by accident (DW: The Two Doctors and Time Crash). Notably, when the Doctor's Fifth and Tenth incarnations met following the destruction of Gallifrey (DW: Time Crash) it threatened to cause damage to the space-time continuum, suggesting a possible rationale for the law (implying the Time Lords likely kept those effects in check during sanctioned" incidents).

One of the Laws of Time stated that an object from a non-existent timeline cannot be present in the current timeline. Cousin Justine of the Faction Paradox had a mask of such description. (EDA: Alien Bodies)

Learning "anything about future Gallifreyan history, causality could be damaged beyond conception...I'm breaking one of the major Laws of Time...It could be the third." -The Doctor (EDA: Alien Bodies)

First Law of Time

 * The First Law of Time was a moral as well as a legal one. (NA: Love and War)
 * The Doctor broke the First Law of Time by leaving messages for himself. (NA: No Future)