The Weeping Angels are a quantum-locked species.
Nicknamed "the Quiet Assassins", a Weeping Angel is stone when observed and cannot move. When unobserved, however, a Weeping Angel can move incredibly fast. A touch from a Weeping Angel displaces you in time, and the Angel feeds on the "potential energy" of the time you would have lived in the present.
- The Weeping Angels' history prior to their appearance in The Game of Rassilon can be found on the TARDIS Data Core.
Three Weeping Angels, one of which was pregnant, were found at a dig site in the year 2533. They found their way onto the UBD Patriach where they attempted to gain access to the ship's Q-MAD drive with the goal of feeding on the energy to nurture the newborn Angels. However they were stopped by the Doctor. Travis, and Millie. (Where Angels Fear To Trek)