A Truth Field was an energy field within which it was impossible to tell a lie. The Doctor encountered one near the bell tower in the town of Christmas on Trenzalore. The field emanated from a residual crack in space-time along with the Question, both transmitted by the Time Lords from outside the universe with the purpose of verifying the Doctor's identity. Being clever, the Doctor could circumvent this; he tricked a group of Ice Warriors by telling them to call him Elias without actually saying that was his name, and on another occasion managed to kill a Wooden Cyberman by telling the Cyberman what he had just done should have reprogrammed its weapon to fire backwards, neglecting to mention the fact that such a reprogramming wouldn't work in the current circumstances.

The Eleventh Doctor told Clara Oswald that he had encountered Truth Fields before, calling it a "quaint" technology. (TV: The Time of the Doctor)

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