My thesis is on the epistemology of the sophist Protagoras. Through the reading of Plato' Theaetetus and Protagoras, I have reconstructed (a) Protagorean theory of knowledge, according to which Protagoras is an inter-subjectivist (as far as perceptions are concerned) and a moral relativist (as far as ethical judgements are concerned).In Chapter 1,1 first try to reconstruct the development of Protagoras' life. I list then Protagoras' few (extant) fragments, offering their different interpretations. Lastly, I deal with modern and most recent scholarship on Protagoras, ending the chapter with some considerations about the scholarly legitimacy of my thesis. In chapters 2 and 3, I deal with the Protagorean section in Plato's Theaetetus. Through a detailed (and critical) analysis of Plato's exegesis of Protagoras' maxim "Man is the measure of all things", I first reconstruct the perceptual (and individualistic) side of Protagoras' epistemology and then the ethical (and collective) side of such an epistemology. At the end of chapter 3, Protagoras' theory of knowledge already reveals itself as a rather complete epistemology. Such a (complete) picture of Protagoras' epistemology is reinforced in chapter 4, which deals with the Great Speech (mainly the myth) of the Protagoras. Through a close analysis of the core of the Great Speech, I confirm the ethical and collective reading of Protagoras' maxim that I have given in chapter 3. I end the chapter by providing some (modern) suggestions for taking Protagoras as a more serious epistemologist than he is actually thought of In the Conclusion, I sum up my whole reconstruction of the Platonic Protagoras and of his theory of knowledge, connecting it briefly with some features of fifth-century B.C Greek epistemology and, again, with some modern philosophical tenets.