A current interpretation of Plato’s reading of Heraclitus maintains that Heraclitus appears in Plato’s dialogues, mostly in Theaetetus, as the main defender of the extreme flux and as the philosopher who asserted an irrevocable interdiction of knowledge and language. However, when we read Heraclitus’ fragments, we can see that he expressly considered the theme of knowledge, that he recognized the possibility of knowledge and language, and that the flux he defended was not extreme. For this reason, this frequent interpretation of Theaetetus produces a kind of uneasiness that could and should be turned into a challenge to the reading of Plato and Heraclitus. Basing its arguments mainly of the first part of Theaetetus, this thesis intends to show how Plato understood and translated Heraclitus thought. One of its conclusions is that in Theaetetus Plato did not transmit an exaggerated image of Heraclitus as a radical flux defender, but rather distinguished Heraclitus’ more moderate theories from his followers’ more radical opinions. If most likely Heraclitus never dissociated the universal flux doctrine from other of his theories, as the doctrine of the unity of opposites, and neither Plato did isolate the theory of flux, we should not consider that Plato imputed to Heraclitus an unilateral and impoverished version of heraclitean philosophy. Finally, contrarily to what may seem at first sight, in Theaetetus Plato condemns only exaggerated heraclitism, suggesting that he accepts and respects the moderate version of the theory of universal flux as proposed by Heraclitus himself.