Laws of Time

The Laws of Time were a set of guidelines and laws guiding what a time traveller could and couldn't do. The Time Lords codified the rules regarding time travel into a set of enumerated Laws. (source?) Beyond the First Law, however, the exact details of these laws were not well understood.

First Law of Time
This Law prevented a Time Lord (or, perhaps any other time traveller), from directly (or perhaps indirectly) interacting with their personal past.

The First Law of Time specifically prohibited a Time Lord from meeting their former selves. (DW: The Three Doctors) Despite this, the Doctor on numerous occasions did just that — either accidentally (DW: Time Crash) or through Time Lord sanction. (DW: The Three Doctors, The Five Doctors) Charlotte Pollard broke the First Law of Time by traveling with the Doctor's sixth incarnation after having been the companion of his eight self, thus exposing the Doctor to his own future. (BFA: Brotherhood of the Daleks) Similarly, the Doctor may be said to have broken the First Law of Time by leaving messages for himself. (NA: No Future)

This was a moral as well as a legal one. (NA: Love and War)

The Doctor once told Rose Tyler that "there used to be laws preventing this sort of thing" in reference to her interference with her own past. However, he failed to enumerate them. (DW: Father's Day) Likewise, the Brigadier's encounter with his past self was described by the Doctor's fifth self as being bad, but not as a specific violation of the First Law. (DW: Mawdryn Undead)
 * It was therefore possible that other laws of time were concerned with the Blinovitch Limitation Effect, the more generalized problem of any being meeting a past version of themselves. 

During his ninth and tenth incarnations, the Doctor willingly caused tiny loops in the timeline of those specific incarnations, without citing a violation of the First Law. Indeed, the Doctor once told Martha Jones that "crossing into established events is strictly forbidden, except for cheap tricks". (DW: Father's Day, Smith and Jones)''

Other Laws of Time
Another 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, a time-aware faction opposed to the Time Lords, which, as their name indicated cultivated and reveled in time paradoxes, had a mask from another timeline. (EDA: Alien Bodies)

It traditionally prevented Gallifrey's 'present' from interacting with its own subjective past or future. (NA: Lungbarrow). In his eight incarnation the Doctor said that learning "anything about future Gallifreyan history" would seriously unbalance the concept of causality. When he proceeded to nevertheless break this law, he claimed, "I'm breaking one of the major Laws of Time...It could be the third." (EDA: Alien Bodies)

Other information
The Doctor stated that he was "...Defender of the Laws of Time" in his seventh and eighth incarnations. (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)