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The Original Trilogy doesn't show any holocrons. We see some in the prequals, in the Jedi Library. We see a lot in the games, and they are mentioned in the books. A lot of the Jedi holocrons were destroyed during the Jedi Purge of Order 66, that happens in Revenge of the Sith. I'm not sure we see Sith holocrons in the prequals, but we do in the animated series The Clone Wars.

Oh I see. Alright thanks!