'You are going to entrust your soul to the care of a sophist. But I should be surprised if you even know what a sophist is.' In the fifth century BC professional educators, the sophists, travelled the Greek world claiming to teach success in public and private life. In this dialogue Plato shows the pretensions of the leading sophist, Protagoras, challenged by the critical arguments of Socrates. From criticism of the educational aims and methods of the sophists the dialogue broadens out to consider the nature of the good life, and the role of pleasure and intellect in the context of that life. The dialogue combines subtlety of argument with intricacy of dramatic construction and brilliant characterization. This translation achieves both precision and colloquial naturalness while the notes and introduction set the arguments in their historical and philosophical context.